I've got a bunch:
Slipstream: Robots, dinosaurs, rocket ships, swords, and lasers in fast-paced, episodic, sci-fantasy action. I'd like to run this as a short (4-10 session) mini-campaign and change of pace from my usual gritty historical fair. My problem is system. "Wiggy's" setting book is superb, but I don't like Savage Worlds much. I would need to re-tool BoL or OD&D or something.
Brithright: I really like realm management type games and this has a great semi-unique fantasy/medieval setting to boot. I've tried to get it off the ground a couple of times but for one reason or another it didn't take off. I should really cross it off my list as the rules just aren't that functional, but I keep tweeking them and coming back.
Stone and bronze: I'd like to run a late stone/age early bronze age type game with a post-apocalyptic twist. This one would include a re-tool of Realms of the Unknown and run as hybrid RPG/Wargame. This one is way on the back burner.
Classic dungeon romp: I'd like to string-together 10 classic modules from low level to high, and run them as is with minimal modification, using 1st ed D&D, just for nostalgia.
I've never run the Great Pendragon Campaign, and that's something I need to do before I die - Monty Python jokes and all.
There are others. Life is too short to play them all.