Here's a quick one. Your standard magical girl game but the general theme is books. The PCs are librarians and patrons and they transform into Sailor Noir, Sailor Romance, Sailor Sci-Fi, Sailor Fantasy, etc. The villains are all manifestations of literary bad guys and monsters. Of course, the City has a sprawling metro library system so the action can happen anywhere in the City where people live and work. Perhaps they all have intra-library loan powers and can teleport from one library branch to the next. If you must have the goofy mascot animal, I'd go with an Owl by the name of Loaner.
You can play with a lot of library terminology, check in and out your powers, renew them. Get you loaning powers suspended. Pay overdue fines. There's the need to be quiet during combat, so a lot of battle whispering. Maybe all the magic powers are described as book titles.
Ah crap, I might as well put this on my New Year's Resolution list as a game/hack/playset to create.