An HD remake of Killing Floor would be pretty sweet, just imagine that on Frostbite 2. Just thinking about that is bringing a smile to my face.
I've also thought any game set in the middle ages and with free roam like GTA would be sweet. You could join gangs, do turf wars, steal horses, hold up shops, and all of this using a sword or a crossbow. Throw in some fantasy stuff and that could be a pretty awesome game, just imagine dog fighting in the skies, but on a fucking dragon!
*Edited to avoid sounding like a douche*