I had planned to make an RP where people could easily join and leave, with a way to explain their absence and presence. It sadly never went anywhere.

The initial idea was a series of puzzle rooms, with the classic "Everyone woke up here with no memory of arriving and now they have to solve puzzles for reasons idk it'll be explained in the sequel" premise. Inactive players (or players who requested to leave) were killed in traps, new players "woke up in" the current puzzle room with everyone else.