Recently a graphic novel came out entitle simply "Joker". Now obviously they can't bring the Joker back in the next movie, but in the book, other villians such as Penguin, The Riddler, and Killer Croc are presented in the same type of "real world" grittiness that the Nolan movies portray, to great effect. I'm not normally a Riddler or Killer Croc guy, but after seeing these incarnations, I'd say just about any character they choose as a villian would be pretty good, as long as it was in the same style. Me personally, I'd like to see a "Batman coming to grips with his own psyche" story, really trying to answer the question of who's the dominant ego, Batman or Bruce Wayne, and which is the mask?