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?
Possibly. The Joker had no plans, but if you really think about it, he kind of did. Y'see, The Joker was kind of a contradiction. That was evident when he spewed two different stories about his past.
He was an enigma, a mystery, perhaps even to himself. He did everything he did in The Dark Knight
to manipulate the world around him and it didn't matter what he did or how he did it. He definitely had plans, despite whatever he spewed to Dent or Batman.