I'd commit grave sins to be able to inhabit and play in Reynolds' Revelation Space universe. The bizarre post-human factions alone, so alien and horrifying in the best way. Could legit make for a really dark MMO. I'd have to go Ultra, though, no question.

The extreme timescales, the highly personal, self-driven body modification, culminating in a truly unique, grotesque sort of personality to one's own body...one which can't help but physically, visibly project the creeping, gibbering paranoia nurtured by millennia spent slowly becoming so estranged from every other lifeform once called kin, making one's journey through time and space utterly, irreversibly alone, even when traveling with others...

For sheer thrill and a tight looter-shooter game, on the other hand, I'd be SO stoked for one using his Revenger universe. Ohhhh to crack those baubles, each a potential Pandora's box of hilariously dangerous relics involving hideous and long-forgotten exotic physics...not to mention, who knows who or what has been lurking just beyond perception, waiting for you to do the risky dirty work of extracting some particularly nasty doodad...

One can dream.