Robin Williams is a good example of someone breaking the mold. He's a great comedian. He was also great in What Dreams May Come, a very serious drama, and Jacob The Liar. Very creepy in One Hour Photo. I can't think of any other actor who can jump through so many opposing genre's so easily. Comedy, thriller, drama, suspense, children's comedy. I'm also hoping he gets his wish to be cast in the next Batman movie.

Granted, you're talking about hero/villain, but the same goes for genre's. There are some actors who absolutely cannot play their opposite. No matter how hard they try, they can't manage it. So to answer your question: I'd love to see it happen, but only if they're not given the part simply because they're well known. In other words, they damn well better be able to play the part.