Voyages in Eternity just completed a five-part series of articles detailing his dream settings for an MMORPG. Its a great read. His dream settings included some of my favorite RPGs: Star Frontiers, Gamma World, and Call of Cthulhu. There is a lot of potential for gaming in those settings; hopefully now that the MMO industry has grown it will start to move beyond the rut of Tolkien-esque fantasy clones.
Now, if I had been asked what my dream setting for an MMO (or just a single player RPG or adventure game) would be, it would definitely be one based on the Doctor Who (and Torchwood) universe. It would be fun to play a Time Agent (like Captain Jack Harkness) and battle iconic Doctor Who villains like the Daleks and Cybermen, and occasionally meeting the Doctor and his Companions in your travels. The beauty of this setting is that it spans all of space and time, so you can tell any kind of story: a macabre horror in Victorian gaslight London, a sci fi space opera, a historical romance, a whodunnit mystery, exploring the unknown on an alien planet, avoiding dinosaurs in the ancient past -- or the modern streets of London!, anything.
I guess the closest thing to that type of openness would be Stargate, because each planet is different from one to another in terms of population, level of technology, and culture. So while I never really got into Stargate as much, I do hope they manage to do that element of the series justice when (and if) the MMO Stargate Worlds comes out.