Mortal Kombat VS. Killer Instinct. I'd love to see the insane, blood-spattered hybrid of the fatality and the UUUUUUUUUULTRAAAAAAAA COOOOOOOOOOOOMBOOOOOOOOOOOOOO!
A Fallout MMO, with Fallout 1, 2, or 3-like gameplay.
A ninja game like Tenchu with platforming like Assassin's Creed and some stealth elements that are similar to Metal Gear Solid 3 (Slow movement, camouflage, etc.) and maybe a way to control which stealth kill you do.
I also liked the idea Yahtzee mentioned in the review for Saint's Row 2, a sandbox game where you get to be a supervillain and terrorize the town with dastardly deeds. Also, fight some superheroes, maybe?