I made a campaign world once in which the planet itself had become uninhabitable, and so everyone lived on a few different moons. Dragons had been domesticated for space flight. Each moon was themed differently (big city moon, constant war moon, undead moon, etc), and adventurers would still head down to the planet to adventure amongst the ruins of ancient civilizations.
I don't think we ever got to play for a long time, but I remember it being one of my first big custom settings after getting into D&D.