Doctor Who
For some reason the idea of a Doctor Who West Marches game keeps popping into my head.
Players can be any version of the Doctor, official or new, or any companions they'd like, from the whole history of the show, including the novels, comics, and audio dramas, or create an entirely new one.
Then just let the players self-organize into groups and go on adventures. One Doctor, multiple Doctors, no Doctor...companions, UNIT, Paternoster Gang, whatever. And weirdly enough, because it's Doctor Who, it would still work.