I read your series today and I really liked them. Your opinions are well supported, and your analysis covers each keyword from many different aspects in a broad view.
I have one correction to make though. Suspend wasn't the first ability to use the exile zone in a way that no other card could, imprint in Mirrodin block did it first (albeit in an unexciting and not very memorable way). This mechanic still has quite a lot of untapped design space.
I don't have an idea what the first keyword would be. Trample is one of the most beloved mechanics of all time, that plays well and has history. However, it's not very innovative - many attacking/blocking mechanic have been made alongside it from the very beginning, and it's not as resonant as flying.
OTOH I definitely know what will make the worst ten keywords of all time: Shroud!
Sorcery — Divination (C)
Divination nexus (This spell costs less for each divination spell played before it this turn)
Draw two cards.
Hostile Land :2mana::symg::symb:
Sorcery — Necromancy (U)
Target player loses 1 life for each land he or she controls.
Flashback :6mana::symg::symb:
Spelltouched Spike :2mana::symg::symg:
Creature — Spike (R)
~ comes into play with four +1/+1 counters on it.
:1mana:, remove a +1/+1 counter from ~: Add one mana of any color to your mana pool.
:2mana:, remove a +1/+1 counter from ~: Destroy target artifact.
And one card, which isn't original but I worry about his power level:
Seismic Bolt :2mana::symr::symr:
Sorcery — Evocation Force (C)
Destroy target land.
Suspend 4—
The idea is a land destruction spell that is supendable at first turn. Is it too strong?
I really like all the cards, but my favorite by far is Sealed Savior.
Yes' it's true that sometimes you want player to kill each other, but sometimes you want to use it for political gain. Say, one guy is countering an akroma. Each player wants you to counter the other player's spell. to remain unaligned you counter both. more to pay for the versatility is a fair price.
And in 2HG it is the most useful. One opponent plays a spell, your teammate responds, the second opponent responds, you double counter. It happens a lot more that you might think.
And in each case, if you need to counter one spell you always can.
And anyway, how can I reduce it cost from :2mana::symu::symu: without making it strictly better than cancel?
I think I can up the warden toughness by 1 if I change it's cost to :3mana::symw::symu:, can't I?
Creature - Snake Shaman Warrior
For each card in your library...
Snow Creature — Elf Druid (U)
:symtap:: Add :symg::symr: to your mana pool.
When ~ comes into play, if you paid to play it, it gains haste.
1/2
Next: A non-dwarf in a dwarven village/city
I know that the cardtype ripple is problematic. I keep that in mind, and make all the necromancies pricier.
Demon Jester's Smile— I thought about using it with null profusion, but there is always use for three extra mana at instant speed.
All the subtypes has reasons behind them.
I agree with doubting dave's idea:
Doucancel :2mana::symu::symu:
Instant —Abjuration (U)
Counter up to two target spells.
Every one thinks that the invisible warden is off color. Maybe multicolor it with white?
Enchantment (R)
At the beginning of each player's upkeep, that player loses 1 life for each creature he or she controls.
:1mana::symb:: Put a 0/1 black Sheep creature token into play under target opponent's control.
"Never trust a farmer who rears black sheep" — Petven, Commoner
Sorcery — Arcane
Forecast —
Next:
1) :xmana::symb::symr:
2):symb::symr:
3):xmana::xmana::symr::symb:
Artifact (R)
:symtap:: Add one mana of any color to your mana pool.
:symw::symu::symb::symr::symg:: ~ becomes a 7/7 multicolored sliver creature until end of turn.
Creature — Spirit
:symtap:: Deal 1 damage to target opponent.
:symb:: Target player who was dealt damage this turn by ~ loses 1 life.
2/1
Next: Auriok Glaivemaster
Sorcery — Conjuration creation (U)
Put a blue artifact card from your hand into play tapped.
Slow Reanimation :1mana::symb:
Sorcery — Necromancy (U)
Choose a creature card in your graveyard. Remove it from the game with three time counters on it. if it doesn't have suspend it gains suspend.
Is it overpowered?
Mana Flight
Artifact (U)
Whenever a land comes into play, add one mana of any color to your mana pool.
Demon Jester's Smile
Instant — Conjuration (C)
Add :symr::symr: to your mana pool.
Hellbent — Add :symb::symb: to your mana pool.
Formchanger Wall :symu::symu:
Creature — Wall (U)
Defender, Protection from colorless.
Whenever ~ blocks, turn all creatures it blocks face down.
1/2
Surging Screams :4mana::symb::symb:
Instant — Necromancy (U)
Destroy target creature. It can't be regenerated this turn.
Necromancy ripple 4 (When you play this spell, you may reveal the top four cards of your library. You may play any necromancy cards revealed this way without paying their mana costs. Put the rest on the bottom of your library.)