The latest rumor about The Dark Knight Rises is that they will combine Dark Knight footage and CGI to bring back Heath Ledger's Joker for a brief appearance. The article does note that it would only be done with his family's consent, which is good, because some might find it to be in poor taste otherwise.
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.