Honestly, I'm probably in a minority on this idea, but based on my concept of the Riddler I've taken from the Adam West show, the 90's cartoon and Forever, he doesn't feel like a villain that could exist in the Batman world Nolan's created, I think it would almost feel rediculous for someone to commit crime in that manner. As much as Joker is bizarre, based on trailers and clips, his crimes are no different than any other mobster and such, just with a slight twist of insanity. I think of core Batman villains, Two-Face, Catwoman, and possibly Penguin are the only others that I'm aware, which could fit in Nolan's world. Catwoman is essentially a femal Batman on the other side, Two-Face is a tragic figure but primarily a normal human being doing normal actions, and Penguin is really more defined like Joker, he could be a mobster that just has quirks that make people think of penguins. Riddler just seems to rediculous, good for comics, campy TV and cartoons, but not Nolan's world. Again though, I don't follow Batman comics so maybe there's a side to Riddler that would match with Nolan's world and could work.