The Star Magic MtG format always seemed to me to be underexplored.
Treachery Magic, a Mafia-inspired variant that hits some of what I'm looking for here
Mostly I like how it plays with:
These could both be taken further. A roleplay-y variant might involve:
I'd probably give B the means to give others victory points secretly.
Letting W win only if all of RGU win, or B win only if none of them do, means that getting the math of "everyone has an equal chance of winning" becomes impossible, but I quite like the absoluteness of it - it forces W to care even about its redeemable enemies and B to be a rotten bastard even to its friends.
Since none of these victory conditions have to do with life going down to zero, you might have to start tearing out the actual rules and assumptions of MtG until you've basically gotten an entirely new game with new cards loosely inspired by Magic. That seems perfectly fine by me.