Okay, now I'm thinking a top down design of a demonic pact that is impossible to pay. So you get extra cards (or some other advantage) for some amount of turns, but at some point you have to sac the enchantment and an opponent gets a big demon token who has come to collect your debt that attacks you each turn (and is maybe indestructible?). I like the idea of racing to win the game until it catches up with you.
Sort of like a Phyrexian Etchings that leaves behind a creature rather than making you lose life? That'd be interesting. How about giving your opponent a hasty, ball lightning-esque demon every turn that begins as a 1/1 and keeps on growing every turn? Or giving him the creature as soon as the enchantment hits and making it grow every time you get the enchantment's effect? If you want a continous creature, you should probably give the demon Shroud or Hexproof to make sure the black deck can't just Murder it.
ninja edit: Having the demon kill you on hit would probably be cool as well. Might as well give it trample to make sure you can't chump-block it forever.
Here's an idea I've been thinking about, with two variations:
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Exquisite Corpse
Enchantment
When Exquisite Corpse enters the battlefield, put a 0/1 black Spawn creature token onto the battlefield.
Spawn creatures [you control] have all activated abilities of creature cards exiled with Exquisite Corpse.
XBB: Exile target creature card with converted mana cost X from a graveyard. Put a number of +1/+1 counters on target Spawn creature equal to that creature's power.
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Another Exquisite Corpse
Enchantment
When Exquisite Corpse enters the battlefield, put a 0/1 black Spawn creature token onto the battlefield.
As long as a creature card with flying is exiled with Exquisite Corpse, Spawn creatures [you control] have flying. The same is true for fear, first strike, double strike, deathtouch, haste, hexproof, landwalk, lifelink, protection, reach, trample, shroud and vigilance.
XBB: Exile target creature card with converted mana cost X from a graveyard. Put a number of +1/+1 counters on target Spawn creature equal to that creature's power.
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While I'd love to fit both abilities in a single card, that'd probably be impossible to do. Given Cairn Wanderer's rules text (adding hexproof to the mix), I think the second card might not even fit by itself. The +1/+1 counters could also be based off of the creature card's CMC, making the final ability
"XBB: Exile target creature card with converted mana cost X from a graveyard. Put X +1/+1 counters on target Spawn creature." This'd probably differentiate it from Scavenge while keeping the power level in check (no exiling Phyrexian Dreadnoughts, Death's Shadows or other undercosted high-power creatures).