Just recently came up with a location concept for a campaign world of mine. The plane involved is essentially a big empty pit that fiends use as a dumping ground. The specific location this post deals with is a prison shaped like a giant capped-off carbon nanotube, like a big hexagon-coated headache medicine floating in a particularly empty bit of void. Now, for the theme behind the occupants:
You know the comic book scenario where a superpowered villain gets a pardon in exchange for using his powers for some corrupt government purpose? This prison is for that sort of thing: All the occupants have exotic powers, and the prison is designed to hold them until the owners need one of them, about once every couple centuries or so.
Needless to say, most of the ones that haven't been put in some form of stasis are in a foul mood. Most are of the "Outsider" type, and more or less immortal, others are constructs, and a few mortals, who are typically held in stasis. The exotic powers, well, that's up to your imagination, but if you need help thinking of something, try perusing one of the various alternate magic systems out there.
Additional critters I could use some ideas for: The constructs that serve as the guards and warden, as well as the owners of the place.