Probably the last big name, Clayface is usually a shape shifting man who looks like a pile of mud in his unaltered state. In addition to being able to look like other people, he can change his shape almost Reed Richards-style, enabling him to be a physical threat to Batman.
But there have been, by wikipedia's count seven Clayfaces, with a few differences. The two most notably different are the original Clayface, Basil Karlo (far left, above), and the third, Preston Payne (far right). Karlo was the star of a classic horror film where he played a villain called Clayface, but when he found out they were remaking it, went nuts, put on the costume from the movie, and started killing people. Payne was a scientist with a disfiguring medical condition and tried to adapt one of the shapeshifting Clayface's powers to cure his own condition, but the experiment resulted in his skin melting when exposed to air and will melt anyone else he touches. Payne builds a suit with what looks like a hood made of plastic wrap to keep himself from melting or touching anyone else, and goes somewhat understandably nuts from the isolation.
Pros: Clayface is a classic villain, visually interesting, and (even though between Founders, Skrulls, and Cylons its becoming a little cliche) not being able to tell your enemies from your friends is both fun and socially relevant.
Cons: The shapeshifting. I love Clayface, but the movies have skipped almost every science fiction element of Batman stories. Suddenly introducing a shapeshifter (or a melting skin dude) doesn't work very well.
How I'd do it: Use Basil Karlo. A guy in a movie costume calling himself Clayface works just fine.
Casting Guesses: Between Ed Wood and Entourage, Martin Landau has shown a tremendous ability to play a has-been, which would be perfect for Basil Karlo.
Verdict: Having a Clayface who isn't at all clay-like would be weird for casual fans of the comics, so it's not very likely, but it could happen.
After The Scarecrow, Ra's Al Ghul, The Joker, and Two-Face, who should be Batman's next enemy in the Batman Begins/The Dark Knight series? Well, as of now we know that Tom Hardy will play Bane and Anne Hathaway will play Catwoman.
Check out the master list for all the villains covered. I've pretty much run out by now. Obviously, there are more Batman villains, but not any of significance that I know of. As of February 2009, no one is confirmed as a villain. Not the Johnny Depp Riddler, not Phillip Seymour Hoffman Penguin, nobody.