Heath Ledger is set in stone as the Joker. I think he's a good actor, but I always thought the Joker should be slightly older than Batman. Christian Bale is five years older than Ledger.
Going back to Two Face though. Billy Dee signed on to Batman '89 because he was under the impression that he would play Two-Face eventually. I think he signed on for multiple movies and that Warner Brothers still gave him a little something to bow out and let Tommy Lee Jones take the part. Personally, I would've loved to see Billy Dee as Two-Face. "How about an ice cold Colt 45,Batman?"