First off a new MMOFPS with two factions trying to take back a city and possibly the surrounding suburban areas. You would have to be able to capture streets, individual buildings, subways, maybe sewers and so on. Ideally I'd love for it to be a destructible or at least semi-destructible environment but I don't think the computing capacity will be there for a few decades... You could set up bases, artillery installations in the suburbs, build helicopter pads, clans could take individual buildings and hold them, stalk through the sewers in small death squads, and so on. I think it would be entirely possible to do a city somewhat like Philadelphia with relatively few tall buildings and lots of row houses where you can create a block by just copying the same model down the street.
I also liked "The Fly" idea and think various animal SIM's could be a lot of fun. You could be a mouse in a house and your job is to collect various things while avoiding traps, pets, people and possibly other mice. You could do the same thing with just about any animal and I would try it out. Hawks, squirrels, snakes, whatever would be quite a lot of fun.
A more free-form RPG similar to Oblivion where you could take on various roles including non-combat ones. Be a fighter, a magic wielder or a merchant. Include dungeons spread out all over the place to explore, multiple cities and towns, quests, and maybe even a few larger-scale quests (but no main quest). I think making it semi-massively multiplayer would be kind of cool so you are in a world with a few friends or up to twenty random people but not a few thousand.
I'm sure I'll come up with more the second I hit post but that's good for now.
Oh, and I support any sequel to Homeworld 2.