I'd like to see a good hard sci-fi game. Traveller just doesn't do it for me, and Star Wars and Fading Suns are really space fantasy, or sci-fi lite. Something along the lines of Transhuman Space D20 would be excellent.
Postapocalyse is a fun genre too. I'm not sure Gamma World is really what I'm looking for, although I'll have to check out Redline after reading Joe Kushner's review.
What I would looooove to see (anyone who works on Spycraft, take note) is a WWII game focusing on OSS members and counter-intelligence. Lotsa intrigue, exotic foreign locales, foiling Nazi plots, breaking up Fifth Column sabotage rings, all that good stuff. I understand Polyhedron did a recent minigame about WWII grunt action, but I'd like something a little more "espionagical".