Re. Lapis Exilis:
Those are some really interesting points. I hadn't really looked at the natural growth of Batman, Gotham and its villains in such detail in Nolan's Batman. But you're right, I think, and I agree that it's making the movies better than the competition (I can't really think of any exceptions).
So, just based on the situation at the end of TDK and the points you've made, wouldn't the Riddler be a good choice? The Joker arguably threw all of Gotham into incredible chaos and the only way to resolve that chaos and madness was for Batman to go along with it to a degree by accepting responsibility where he's not responsible at all.
With the chaos and the opening left in the underworld wouldn't the Riddler be a pretty natural development? He would seem similar to the Joker at first as his schemes would not at first be apparent. But it would become clear - at least to a character such as Bruce Wayne / Batman - that there's actually a clever mind with a plan at work. However, this would be plan you could actually figure out in some way.
So, on the outside, the Riddler would continue some of the madness and chaos but, really, it wouldn't be like that at the core.
Wouldn't that be a possible direction to take?
My problem with Riddler is that he simply isn't enough of a threat to Gotham as a whole. He's always been an adversary focused on one upping Batman, or showing off his own intelligence (except of course in the memorable and wonderful Dark Knight, Dark City
three-issue arc - but that wasn't really the Riddler). I can see Riddler and a bunch of others showing up in various guises to fill the power vacuum in Gotham's criminal underworld, but I don't see how he challenges the character dilemma enough to be the main threat - unless as was talked about earlier in the thread.
I could see a story in which the escalation theme is carried forward - crazies move into the power vacuum left by the mob leadership being killed. So the other side escalates. A politician or law enforcement leader (FBI or the state's AG) decides that harsh methods must be used to establish order in Gotham, instituting what amounts to martial law, and which includes a full on manhunt for the vigilante. I could see some uses of Hugo Strange in this area, as a profiler brought in to identify the Batman. I could see Riddler or some other Rogue used as a tool to enact chaos, thus justifying even more government control. Gordon and Batman find themselves at first pleased, and then horrified. It would make for a really interesting companion piece to TDK. It gives room to further explore the personal and philosophical dilemmas that have characterized the series. It would fulfill the requirement that Batman works best in a world that's gone to hell. It would follow naturally out of the events of TDK. It could be scary as hell to see a world brought to life where someone uses the Batman's own tactics to control crime taken to an extreme. Now it's not just one vigilante breaking the law, but an entire secret police force controlling the population completely.