I think most fans assume that there's no way Nolan will be able to top The Dark Knight when he finally steps up to direct his third (and final?) Batman movie. There's been some pretty wild speculation and rumors about who the next villain or villains might be (there always is, with every Batman film since 1989), but the Riddler is the only member of Batman's rogue's gallery that would elevate the third Batman film into something truly remarkable. Nolan would get to make his usual, muscular Batman movie, but with all of the intricate, corkscrew twists and turns of his puzzle-box thrillers.
Go ahead and clear your mind of the memory of Jim Carrey prancing around with a neon blender on his head. Forget about Frank Gorshin's giggling jokester in apple-green tights. Nolan would be just the man to bring the Riddler to life on the big screen in a way that wouldn't just make for a great Batman film -- it could very well revitalize the character in the comics as well. Too many Batman writers treat the character as an annoying, wormy pest as opposed to the legit threat that he could be.
My bit of fanboy dream casting? Sam Rockwell. I'd produce the film myself, right now, if I could.