Here's my attempt at a Whodunnit? mechanic, inspired by Murders at Karlov Manor.
Choosing suspects works exactly like choosing a ring-bearer (except three creatures.) You would use the provided double-sided counters to mark your suspects. Look at them secretly, then place them on their "?" side. Then reveal each when it's most convenient.
Like ring temptation, concealing a new crime resets your suspects and all suspect marks.
Storm the CastleRWG
Battle - Rescue (R)
Save a Noble card with mana value 3 or less (A rescue can be your commander. A commander rescue starts the game in the battlefield, with a card from your deck that fits the condition in exile. A rescue is protected by your opponents.)
When you defeat Storm the Castle, if it's your only commander, put the card to be saved in the battlefield and it becomes your second commander. Otherwise, search your library for a Pirate, Soldier, or Warrior card, reveal it, and put it into your hand. Shuffle afterwards.
5
You can probably guess the inspiration for Storm the Castle. In commander, having to rescue your commander would push towards more aggresive decks. Probably just a gimmick, but a fun one. You could rescue bigger creatures, putting them into your hand rather than the battlefield, since the resolution of the rescue is part of the card's defeat condition.
For Kamigawa:
Jungle Cyb-Arena2GU
Battle - Network (U)
(As a Network enters, choose an opponent to protect it. You and other players can attack it, but damage doesn't remove defense. Instead, the first time each turn its damage reaches its defense, hack it, then put a defense counter on it.)
When Jungle Cyb-Arena enters the battlefield, create two 2/2 green Spirit Wolf creature tokens.
Hack - Put a +1/+1 counter on target creature. Draw a card.
3
I wanted to use "firewall counters" rather than defense counters, since they work differently (and cooler flavor!), but I think the default wording is shorter and reads better. These battles would represent Cyberspaces or corporate servers, with a "hack the mainframe" flavor. Each time you hack it, it gets harder, so you may get tons of value... but then ask yourself how many times over you would have won with damage.
Orzhov Spy(B/R)(B/R)(R/W)(R/W)
Creature - Human Rogue Cleric (U)
When Orzhov Spy enters the battlefield, check how much R you spent to cast it -
No R - Put a lifelink counter on it.
Some R - It gains haste until end of turn.
All R - It also gains trample and gets +2/2 until end of turn.
4/4
Golgari Spy(U/B)(U/B)(G/U)(G/U)
Creature - Human Rogue Warlock (U)
When Golgari Spy enters the battlefield, check how much U you spent to cast it -
No U - Destroy target creature an opponent controls.
Some U - Draw a card.
All U - Also tap target creature an opponent controls and put two stun counters on it.
3/3
The idea here is that a rakdos+boros colaboration could make the perfect orzhov spy. At least until the ceremonial wine gets passed around.
This seems like a good starting point for how these cards might end up working.
The biggest challenge I see with designing and balancing these effect is making the pay off worth the hoops you jump through to get it. In the above, for instance, is the 2 cards and a utility land worth paying 4 mana and then having to send creatures off from attacking my opponent?
That's what I tried to address with the "asymmetry."
You don't lose damage for sending into battle, but your opponent does. Converselly, you only gain 1 initiative per connecting creature, but your opponent removes the full amount of damage (that wasn't dealt.)
I think the best way to see it is the Floor and Ceiling of the card (when it's at worst, and at best.)
At worst: You have no creatures. Your opponent ignores the battle completely. This is the same floor of an Equipment/Vehicle, though. At worst, but you have a single creature: You send the creature into battle. You can win in 4 turns unless the opponent sends something too (or removes the creature: see above.) If the opponent sends a creature, you eventually get the LOSE trigger, and saved some life. At best: You have four fliers. They get +1/+1 this turn, attack for full damage, and you immediately win the battle. Pretty powerful for 2UU and why you included this card in your deck, even if it was quite conditional.
In the middle, you could expect to get a bit of a push and pull. How many creatures does your opponent send? I forgot to rule this, but I'd have them commit the creatures before you do, which is quite the upside on a big battlefield.
With the new card type coming in a future set, I'm trying to come up with a good design for a battle, that doesn't fall into the typical win-more pitfalls. Here's what I came to...
Quick Rules: Any creature may enter a Battle when either enters the battlefield. Creatures in the battle may only block other creatures in the battle. Damage from unblocked creatures adds or removes initiative counters. If a battle reaches 0 initiative, it's lost. If it reaches its goal number, the battle is won.
However, damage done in a battle is not symmetrical.
- Damaging the controller of the battle removes that much initiative INSTEAD of the damage.
- Damaging other players through the battle adds only 1 initiative, but deals full damage.
Design considerations: Winning is generally better than losing, but the difference shouldn't be huge. If you facing a huge disadvantage, spending 4-5 mana in a battle shouldn't be the end of the game after the next combat step. It should at least divert the opponent, much like a planeswalker may do. This is the main reason the battle conditions are asymmetrical.
Full Rules:
As a battle enters the battlefield, players may move any number of their creatures to its 'zone'.
As a creature enters the battlefield, its controller may have it enter a battle.
As a noncreature permanent becomes a creature, its controller may have it enter a battle.
A battle enters with a number of counters equal to its starting initiative (first number in S/G, or "Start/Goal")
If a creature in a battle would deal combat damage to the battle's controller, it removes an initiative counter INSTEAD.
If a creature in a battle would deal combat damage to another player, it adds an initiative counter AND deals damage.
If a battle's initiative reaches 0, it's lost. If it reaches the goal initiative (second number in S/G) it's won. In both cases the battle is sacrificed, unless it transforms.
Example
Battle of the Peaks2UU
Battle
Flying creatures at this battle have +1/+1.
Win - Draw two cards and transform Battle of the Peaks.
Lose - You may discard a card. If you do, transform Battle of the Peaks. (Otherwise, sacrifice it.)
4/8
-- Windy Peaks
Land
(Transforms from Battle of the Peaks) T: Add U. 3U, T: Create a 1/1 blue Bird creature token. Activate only as a sorcery.
Well, that's certainly the more convential route, but I wanted to break new ground. These are like 8-balls that you may shake and shake until you get the result you want/need. But since they must tap to get the result, you are still limited to 1 effect per turn.
The choice of advisors to activate is a way to create advise-synergy, and I think it's flavorful that, if you have many people giving you advice, you'd be able to pick the one that suits you the most.
Asking for advice is random by design: you never know what you are going to get. You can keep trying until you get the advice you want, but you may spend tons of mana with no luck and nothing to show for it. Still, if you have plenty of advisors, you can pick the advise you prefer, then tap that permanent to get it. Bonus if the asking price was cheap, and the reward expensive.
This is a mechanic that allows you to abandon a color. In Ravnica flavor, you are burning your bridges with your guild. You lose access to half your colors, but not your expertise, and you could find a more painful but valuable workaround.
Mechanically, forfeit is great for splashing, mixing Mana fixing and Mana sinkage. In the late game, playing spells of a color you have forfeit can refill your hand, but you better be able to spare the extra mana and life. In the early game, if you are missing a splash color, you can forfeit that color and pay it as 1, but the rest of the splash will be delayed for much later turns.
On the color pie 'break', cards that forfeit must work as any of their colors (gruul defector couldn't destroy enchantments since it could be monored.) But after that, changing a color to colorless (but 3 mana more expensive) is very much in line with any number of mana rocks, so think of forfeiters as colored inefficient mana rocks that enable another color.
A mechanic to represent star-crossed lovers (who could be Romeo and Juliet in UB promos.)
You start the game with both in the command zone (like regular partners), but if you play Ava, Loretto turns into his conspiracy. You need to hit your adversary (with mana open) to rescue him. Then you cast Loretto, but now Ava turns into a conspiracy (assuming she is revealed in a revealed zone.) Conspiracies always exist in the command zone, so there she goes.
Turns out, Ava was the ringleader of the organization Loretto was spying on! What drama!
A mechanic for ancient slumbering versions of current creatures. Tremorract is kinda unpowered, but still an uncommon which would make some decks. A more powerful rare version could cost 1RR normal, and have ancient 3R, dormant 2.
Swift Silentwing1U
Creature - Owl(R)
Flying
Dormant 3 --2U-- 5/6
Rumble - 3U: Draw a card (Activate only as a sorcery and only once each turn.)
2/1
The set would come with Fossil punch-out tokens, with an "Awoken" reminder on the back, so you use 3, then flip the last one to prevent memory issues (if you somehow forget the 9/9 that took 3+ turns to arrive.)
Interesting, but that would cycle quite quickly. How about using Join Forces.
The Drake's SongUW
Sing-along (As this saga enters and after your draw step, starting and ending with you, each player may pay 1 or 2 mana. If 3 or more mana was paid, put a lore counter. Each opponent that paid also triggers the current chapter. Sacrifice after III.)
I - Draw a card and gain 2 life.
II - Create a 2/2 blue Drake creature token with flying.
III - Put a +1/+1 counter on each creature you control.
Sing-along is a group hug commander mechanic. You gain verse counters like energy, then spend it on the Saga's final chapter. On the way there, you choose which opponents get access to which chapters, but they may shut you down by not asking.
In 2v2, these are effectively 2-chapter sagas (it'd have to be a weird situation for you and your opponent to accept the colaboration.) But in a stardard set with verse counters as a sub-theme, these could work in that theme.
Phyrexian Rager, in its two iterations. I don't know why. Maybe it was the awesome art, the feeling of "finally, it's all out war" from the invasion, or casting it and feeling the board development without hand advantage degradation.
Choosing suspects works exactly like choosing a ring-bearer (except three creatures.) You would use the provided double-sided counters to mark your suspects. Look at them secretly, then place them on their "?" side. Then reveal each when it's most convenient.
Like ring temptation, concealing a new crime resets your suspects and all suspect marks.
An opponent protects the battle, you protect the planeswalker. Just a fun design for casual play.
Battle - Rescue (R)
Save a Noble card with mana value 3 or less (A rescue can be your commander. A commander rescue starts the game in the battlefield, with a card from your deck that fits the condition in exile. A rescue is protected by your opponents.)
When you defeat Storm the Castle, if it's your only commander, put the card to be saved in the battlefield and it becomes your second commander. Otherwise, search your library for a Pirate, Soldier, or Warrior card, reveal it, and put it into your hand. Shuffle afterwards.
5
You can probably guess the inspiration for Storm the Castle. In commander, having to rescue your commander would push towards more aggresive decks. Probably just a gimmick, but a fun one. You could rescue bigger creatures, putting them into your hand rather than the battlefield, since the resolution of the rescue is part of the card's defeat condition.
For Kamigawa:
Jungle Cyb-Arena 2GU
Battle - Network (U)
(As a Network enters, choose an opponent to protect it. You and other players can attack it, but damage doesn't remove defense. Instead, the first time each turn its damage reaches its defense, hack it, then put a defense counter on it.)
When Jungle Cyb-Arena enters the battlefield, create two 2/2 green Spirit Wolf creature tokens.
Hack - Put a +1/+1 counter on target creature. Draw a card.
3
I wanted to use "firewall counters" rather than defense counters, since they work differently (and cooler flavor!), but I think the default wording is shorter and reads better. These battles would represent Cyberspaces or corporate servers, with a "hack the mainframe" flavor. Each time you hack it, it gets harder, so you may get tons of value... but then ask yourself how many times over you would have won with damage.
Creature - Human Rogue Cleric (U)
When Orzhov Spy enters the battlefield, check how much R you spent to cast it -
No R - Put a lifelink counter on it.
Some R - It gains haste until end of turn.
All R - It also gains trample and gets +2/2 until end of turn.
4/4
Golgari Spy (U/B)(U/B)(G/U)(G/U)
Creature - Human Rogue Warlock (U)
When Golgari Spy enters the battlefield, check how much U you spent to cast it -
No U - Destroy target creature an opponent controls.
Some U - Draw a card.
All U - Also tap target creature an opponent controls and put two stun counters on it.
3/3
The idea here is that a rakdos+boros colaboration could make the perfect orzhov spy. At least until the ceremonial wine gets passed around.
That's what I tried to address with the "asymmetry."
You don't lose damage for sending into battle, but your opponent does. Converselly, you only gain 1 initiative per connecting creature, but your opponent removes the full amount of damage (that wasn't dealt.)
I think the best way to see it is the Floor and Ceiling of the card (when it's at worst, and at best.)
At worst: You have no creatures. Your opponent ignores the battle completely. This is the same floor of an Equipment/Vehicle, though.
At worst, but you have a single creature: You send the creature into battle. You can win in 4 turns unless the opponent sends something too (or removes the creature: see above.) If the opponent sends a creature, you eventually get the LOSE trigger, and saved some life.
At best: You have four fliers. They get +1/+1 this turn, attack for full damage, and you immediately win the battle. Pretty powerful for 2UU and why you included this card in your deck, even if it was quite conditional.
In the middle, you could expect to get a bit of a push and pull. How many creatures does your opponent send? I forgot to rule this, but I'd have them commit the creatures before you do, which is quite the upside on a big battlefield.
Quick Rules: Any creature may enter a Battle when either enters the battlefield. Creatures in the battle may only block other creatures in the battle. Damage from unblocked creatures adds or removes initiative counters. If a battle reaches 0 initiative, it's lost. If it reaches its goal number, the battle is won.
However, damage done in a battle is not symmetrical.
- Damaging the controller of the battle removes that much initiative INSTEAD of the damage.
- Damaging other players through the battle adds only 1 initiative, but deals full damage.
Design considerations: Winning is generally better than losing, but the difference shouldn't be huge. If you facing a huge disadvantage, spending 4-5 mana in a battle shouldn't be the end of the game after the next combat step. It should at least divert the opponent, much like a planeswalker may do. This is the main reason the battle conditions are asymmetrical.
Full Rules:
As a battle enters the battlefield, players may move any number of their creatures to its 'zone'.
As a creature enters the battlefield, its controller may have it enter a battle.
As a noncreature permanent becomes a creature, its controller may have it enter a battle.
A battle enters with a number of counters equal to its starting initiative (first number in S/G, or "Start/Goal")
If a creature in a battle would deal combat damage to the battle's controller, it removes an initiative counter INSTEAD.
If a creature in a battle would deal combat damage to another player, it adds an initiative counter AND deals damage.
If a battle's initiative reaches 0, it's lost. If it reaches the goal initiative (second number in S/G) it's won. In both cases the battle is sacrificed, unless it transforms.
Example
Battle of the Peaks 2UU
Battle
Flying creatures at this battle have +1/+1.
Win - Draw two cards and transform Battle of the Peaks.
Lose - You may discard a card. If you do, transform Battle of the Peaks. (Otherwise, sacrifice it.)
4/8
--
Windy Peaks
Land
(Transforms from Battle of the Peaks)
T: Add U.
3U, T: Create a 1/1 blue Bird creature token. Activate only as a sorcery.
Here is a new (hopefully more clear) version.
The choice of advisors to activate is a way to create advise-synergy, and I think it's flavorful that, if you have many people giving you advice, you'd be able to pick the one that suits you the most.
Asking for advice is random by design: you never know what you are going to get. You can keep trying until you get the advice you want, but you may spend tons of mana with no luck and nothing to show for it. Still, if you have plenty of advisors, you can pick the advise you prefer, then tap that permanent to get it. Bonus if the asking price was cheap, and the reward expensive.
This is a mechanic that allows you to abandon a color. In Ravnica flavor, you are burning your bridges with your guild. You lose access to half your colors, but not your expertise, and you could find a more painful but valuable workaround.
Mechanically, forfeit is great for splashing, mixing Mana fixing and Mana sinkage. In the late game, playing spells of a color you have forfeit can refill your hand, but you better be able to spare the extra mana and life. In the early game, if you are missing a splash color, you can forfeit that color and pay it as 1, but the rest of the splash will be delayed for much later turns.
On the color pie 'break', cards that forfeit must work as any of their colors (gruul defector couldn't destroy enchantments since it could be monored.) But after that, changing a color to colorless (but 3 mana more expensive) is very much in line with any number of mana rocks, so think of forfeiters as colored inefficient mana rocks that enable another color.
A mechanic to represent star-crossed lovers (who could be Romeo and Juliet in UB promos.)
You start the game with both in the command zone (like regular partners), but if you play Ava, Loretto turns into his conspiracy. You need to hit your adversary (with mana open) to rescue him. Then you cast Loretto, but now Ava turns into a conspiracy (assuming she is revealed in a revealed zone.) Conspiracies always exist in the command zone, so there she goes.
Turns out, Ava was the ringleader of the organization Loretto was spying on! What drama!
To make this, I created the 3/3 first, then pasted over it using GIMP a second version 9/9 with the extra elements.
A mechanic for ancient slumbering versions of current creatures. Tremorract is kinda unpowered, but still an uncommon which would make some decks. A more powerful rare version could cost 1RR normal, and have ancient 3R, dormant 2.
Swift Silentwing 1U
Creature - Owl(R)
Flying
Dormant 3 --2U-- 5/6
Rumble - 3U: Draw a card (Activate only as a sorcery and only once each turn.)
2/1
The set would come with Fossil punch-out tokens, with an "Awoken" reminder on the back, so you use 3, then flip the last one to prevent memory issues (if you somehow forget the 9/9 that took 3+ turns to arrive.)
The Drake's Song UW
Sing-along (As this saga enters and after your draw step, starting and ending with you, each player may pay 1 or 2 mana. If 3 or more mana was paid, put a lore counter. Each opponent that paid also triggers the current chapter. Sacrifice after III.)
I - Draw a card and gain 2 life.
II - Create a 2/2 blue Drake creature token with flying.
III - Put a +1/+1 counter on each creature you control.
Sing-along is a group hug commander mechanic. You gain verse counters like energy, then spend it on the Saga's final chapter. On the way there, you choose which opponents get access to which chapters, but they may shut you down by not asking.
In 2v2, these are effectively 2-chapter sagas (it'd have to be a weird situation for you and your opponent to accept the colaboration.) But in a stardard set with verse counters as a sub-theme, these could work in that theme.