I'd love to see how beautiful games like Uncharted 2 and Killzone 2 would be on the PC. The PS3 seems to be always surprising me with what it can manage on a lowly 7800GTX-like GPU, it'd be great to see those titles in higher resolutions with better textures, they're already fantastic.
Also, if Killzone 2 multiplayer were on the PC i bet it would go down a storm. It was way less forgiving than the more popular call of duty franchise and had far more subtle complexities, tbh i think the near-complete lack of autoaim scared off the majority of console gamers instantly as they couldn't play as well as they did in Call of Duty 4.