So the first step is to hunt down every name I can find associated with the Riddler for a future Batman movie. Please remember that this is not a list of my suggestions, because I happen to think some of these are downright awful. And if there are any names you think should be on the list, let me know in the comments.
Updated the list with updates from the comments (though I trimmed some off of Commissioner Gordon's list at the bottom of this post because the polls would never end with that many actors, and, no offense, Harrison Ford would be awful):
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.