I'm trying my hand at a Planescape PbtA hack right now, loosely based on Legacy, that is, players play both characters AND the faction s/he belongs to in this competition for "The meaning of the multiverse". The faction playbooks will be the original Sigil ones, and for character playbooks Im ditching the classic D&D classes in favor of archetypes or concepts more specific to the setting (say, The Fiend, The Planewalker, The Chant Broker, The (Cranium) Rat Gestalt, etc). The expected play structure has players colaborating against threats to Sigil status quo, while doing their secret war to impose their faction's outlook on the multiverse.
I'm stuck on a couple points though:
1) Advancement: I want to use Keys (from TSOY) somehow because I think it's perfect to capture the setting reality of "Belief is power", and nothing more empowering than getting rewarded by putting your belief to practice. These Keys could come from the faction playbook I think (so an Athar character would have some keys like "gain XP each time you kick an idol in front of it's believers" or something). But I'm not sure this would be the best approach. I only know 2 PbtA games which use Key-like advancement: Sagas of the Icelanders (where you gain XP after using a move on each of your relationships) and The Sprawl (where you gain XP by jeopardizing the mission for different reasons). I like the former but the later never worked for us. What other key-like advancements are there in PbtA?
2) Tags. I don't know how to call this yet, but I would like some kind of "label" or "conditions" system that affect characters, planes, etc and could be played with. So when your chaotic Anarchist steps into Nirvana (the plane of order) the conflict of the plane "law" tag with his own "chaotic" gets him a "shaken" condition that makes him weaker for the time being (say, rolling with disadvantage or getting shut down on further advancement). The same could happen as a result of a Seduce or Manipulate move.
Also, I'm thinking the ultimate prize for a faction that wins the current Era race for the meaning of the multiverse could be imprinting his views on reality, thus giving it a temporary "tag" that would also affect people and planes in that manner.
FAKE EDIT: Or maybe it's simpler to forget about tags altogether and just give each plane it's own move like "When you enter Nirvana, get -1 ongoing except if you belong to a lawful-aligned faction". AND also give each faction it's own special move for whe it wins the belief race?