MTG Salvation

Stolen Gifts 3U
Sorcery
Search target opponent's library for up to four cards and separate them into two piles. That player puts one pile into his or her graveyard and exiles the other.

Marchesa's Preacher 2W
Creature: Human Advisor
At the beginning of your end step, if you're the monarch, create a 1/1 white Human creature token.
"Marchesa's underlings are already swaying the people to her side."
-Adrianna, Captain of the Guard
1/4

Risen City 1WWU
Creature: Elemental
At the beginning of your upkeep, each player draws a card.
"When the Orzhov contract the Izzet to animate and re-purpose districts of Ravnica, the Azorius make sure that the elementals serve a purpose in maintaining the peace.
5/5

Stormborn Devourer 2R
Creature: Elemental
Flying, Haste
Cast Stormborn Devourer only if you've cast another spell this turn.
At the beginning of the end step, sacrifice Stormborn Devourer.
4/4

Next:
Jareth, Leonine Titan in B
Thoughtseize in R
Pia Nalaar in W

Artifical Might 5GG
Enchantment
Artifact creatures you control get +1/+1 for each other artifact you control.

Next: Nefarox, Overlord of Grixis

The Nine Titans 1WUBRG
Planeswalker - Urza Freyalise
+2: Draw two cards.
-3: Destroy target permanent.
-X: Destroy each other nonland permanent with converted mana cost X or less.
The Nine Titans can be your commander.
{[5]}

Yeah this is the only proper old school Planeswalker team up I could think of where everyone was dead. I only put Urza and Freyalise down for the planeswalker types since they're the two most iconic names in the titans and one of them is already a walker. Also commander because we all know a true superfriends EDH would have a planeswalker leading them.

Next: Fblthp, Planeswalker

Well firstly Bestow only really works on Enchantment creatures since it turns the spell into an aura (which needs the enchantment type). And I think people have much better things to do with four mana every turn than buff this guy to be a decent card, because it's very weak overall. Even if each ability costed one mana and gave +1 I still don't think it'd be strong enough.. The bestow cost is too mana intensive to make it worthwhile too I feel, most people would just not know what to do with it.

Here's a big card for a set involving counters:

Curse Collector 3BB
Creature: Elemental {R}
When Curse Collector enters the battlefield, remove all counters from all permanents. Each player loses life equal to the number of counters removed this way from permanents he or she controls.
3/3

Parchment Palisade WU
Artifact Creature: Wall
At the beginning of your upkeep, draw a card, then discard a card.
If a source an opponent controls would deal damage to you or another permanent you control, you may have it deal that much damage to ~ instead.
0/3

A 0/4 for two mana that you want to die is extremely good considering it stops aggressive creatures enough for you to fill your graveyard. I second the notion that Gravemorph is grossly overpowered. If you want to go further in the design, then I suggest staying far away from big creatures with Gravemorph you could easily cheat into play by sacrificing a small critter. I reckon making the bones a 0/3 or maybe even a 0/2 to make sure other strategies can possible be more of a threat for the early game.

Here's a very simple white uncommon who although doesn't really impact the rest of the cards in the set, is a flavour win and a definite playable card outside of its own set.
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Tombstone Husher 2W
Creature: Spirit {U}
Cards in graveyards lose all abilities.
Some grave wardens desperately try to grant the rest the dead so dearly deserve and so often roused by explorers.
1/3

It seems really powerful as a common. Getting a free land drop every turn from something like Evolving Wilds enables ridiculous ramp strategies. The first ability I think is fine, but the second one needs a higher cost to pay or a rewording to make your lands playable from the graveyard this turn, or bump it up to a rare.

Yarox, Bladewing's Pride 3(U/R)(U/R)(U/R)
Legendary Creature - Dragon [R]
Flying, Haste
Whenever Yarox, Bladewing's Pride deals combat damage to a player, choose one:
- Draw a card.
- Return target creature an opponent controls to its owner’s hand.
The heirs of Shiv shall always honor the fighting pits.
4/4

Got majorly Nathed here, always forget to go to the latest page

Ashia, Soulsinger WWW
Planeswalker - Ashia
+1: Exile target card in a graveyard. If a creature card is exiled this way, create a 1/1 white Spirit creature token with Flying.
-2: Create three 1/1 white Spirit creature tokens with Flying.
-5: Destroy all non-Spirit creatures. You gain life equal to the number of creatures destroyed this way.
[2]

EDIT: I also noticed that Severance's post got skipped, so here's his elemental walker (Flamekin are Elementals, so I went with that):

Embry of Flame's Path 4R
Planeswalker - Embry
+1: Add RR to your mana pool.
-1: Put a creature card from your hand onto the battlefield. It gains haste. Sacrifice it at the beginning of the next end step.
-7: You get an emblem with "Whenever a creature you own dies, exile it instead, then return it to your hand at the beginning of your next upkeep."
[4]

Next:
A RWB Compleated Koth

I feel like the wording on Aetherial Animator won't allow it to do its first ability. Not sure how you could reword it other than using the actual " until end of turn, target vehicle becomes a creature bla bla bla". Also I'm not sure whether the ability is all that good considering it only makes it a little harder to kill in combat. Definitely fun for limited although a lack of rarity makes that hard to evaluate.

____

Coliseum Grounds
Legendary Land {M}
Coliseum Grounds enters the battlefield tapped.
T: Add C to your mana pool.
3, T, Pay 1 life: Target creature you control fights another target creature. Activate this ability any time you could cast a sorcery.
"The Cabal may be gone, but the fighting pits have endured."

Alvira, Slave of the Hive 2GU
Planeswalker - Alvira Sliver

+1: Create a colorless 1/1 Sliver creature token.

0: Alvira, Slave of the Hive becomes a 4/4 creature with Hexproof until end of turn. Prevent all damage that would be dealt to it this turn.

-7: For each creature you control, create a token that's a copy of that creature, except its a Sliver in addition to its other types.

3

Lore bonus: A shapeshifting planeswalker that transformed into a Sliver, only to become a slave to their hive-mind and transfer his spark to the entirety of the hive = a hive of planeswalking slivers.
__

Next: A Planeswalker card that actually has two characters in it.

It's a very strong card, but I think you should either change the devotion requirement or its mana cost, as two Pyropterix can create an infinite loop which is broken (it gets you an unkillable blocker as well as an infinite amount of ETB/Dies triggers so this card would just be the centerpiece of an aristocrats deck for sure).

Crystal Lizard GU
Artifact Creature: Lizard {U}
When Crystal Lizard dies, draw three cards.
At the beginning of your upkeep, exile Crystal Lizard.
1/1

Seems like a lot of abilities for a very reasonably costed creature. 5/2 trample lifelink makes it a very hard card to deal against, but the free mini lightning helix when it sacrifices an artifact is just bonkers (not to mention the wording of the ability which allows you to bring back the artifact you just sacrificed).

Speaking of way too many abilities, here's a new addition to my custom Mirrodin Pure set:

Atraxa, Praetor's Bane 1WWWW
Legendary Creature - Angel {M}
Flying, First Strike, Vigilance, Lifelink
When Atraxa, Praetor’s Bane enters the battlefield, remove all counters from all permanents and players. You gain 2 life for each counter removed this way.
Her light shines the way to a brighter tomorrow.
4/4

Sigil of Sanctity 3
Artifact - Equipment
Prevent all damage that would be dealt to and dealt by equipped creature.
3: Untap equipped creature.
Equip 2

Next: Ludevic, Necro-Alchemist

Like you said, it seems overly complicated, but gimmicky enough for it to work. You could probably exile more than one card and do a random reveal so you don't have to blink it every time.

Here's my first Walker submission. Hopefully it's comfortably good.

Minadlra, Rune Maniac 1RR
Planeswalker - Minaldra {M}
[+1] Look at the top card of your library. If it’s an instant or sorcery card, reveal it and put it into your hand.
[-2] Minaldra, Rune Maniac deals 3 damage divided as you choose among any number of target creatures and/or players.
[-5] You get an emblem with “Whenever you cast an instant or sorcery spell, copy that spell. You may choose new targets for the copy.”
Starting Loyalty: [3]