Did you happen to see him in Out of Sight? There's something about that performance that tells me he could do it.
I'm basically thinking about actors not normally on the radar screen. Kind of like what Butter said about Heath Ledger and not knowing Heath had the Joker in him. He really tapped into something dark. Who else could do that and surprise the hell out of everyone (instead of making it cartoonish like Jim Carrey, Jack Nicholson, etc.)?
After seeing Half Nelson and Lars & the Real Girl, Ryan Gosling is definitely at the top of the list for me. Have been trying to come up with a female option, too, but am finding it hard to think of anyone.