Girl frame campaign idea
Idea: have each player make a character in whatever TTrpg they want. Have a one on one session with each of them discussing backstory and stuff they want to see in the world. Call it a session -1.
For session 0 have every body introduce their character and run a quick scene where the world ends.
After all those scenes everybody wakes up as their girl frame girls. (You can have them make the girls now or if you know them we'll enough hand them their characters). Insist the other RPG world never happened and you're done indulging their delusions.
Void tainted areas contain a jumble mix of the worlds you created with the girls in the session -1 . Harpies resemble characters from those worlds.
Give them a remember who you are move that if they succeed let's them do stuff from their original character for a scene in exchange for void.
If/when the cadre falls to the void you can continue the adventure in the worlds you initially created.