I tried to not comment on this anymore, but this is killing me. For a character that's been around as long as the Joker has, it amazes me that so many people seem to think he's "finished." That Nolan constructed the only meaningful Joker story in TDK, and that anything else can't possibly be good, or even necessary. Then in the same breath, they bring up all these second-tier villains who aren't even that interesting in the comics themselves to begin with, and actually think they'll make for a better movie?
The Joker's whole point in TDK, much like it was in The Killing Joke, was to prove that all people, including Batman, are fallible and corruptible. Now, news is out that Batman is on the run for murder, which to the Joker means only two things. Either, Batman really did kill someone and the Joker wins, or two, Batman has to take the wrap for Dent and the Joker wins morally, seeing as how he told Batman how quickly the cops would turn on him, given the opportunity. Regardless, the Joker would be very inclined to escape to confront him once more. We're talking about a criminal who wasn't even locked up for a full night in the last film, a mastermind who had schemes set up all over the place to toy with the cops, the mob, and Batman. You mean to tell me that now, all of a sudden, he decides "that's enough!" and sits in his cell peacefully looking at the walls?
The Joker & Batman's feud is an infinite one. If Mr. J were that easy to dispose of, Batman would've been free from his treachery 60 years ago. Nolan recognized this, and probably left him alive to leave that door open, if he chose to do a third. (which I think was pretty much mapped out the whole time but I digress...) Point being, the character of the Joker is everlasting, full of potential, and not locked into any strict characterization. Just because Ledger played it one way, doesn't mean someone else couldn't come in and do something very different, while still being faithful to the things established in TDK. Someone brought up the idea of another Gotham Knights animated bit that addresses the Joker's whereabouts, which I feel is a great idea. But if Nolan somehow decided that he wanted the Joker back for the next film, I wouldn't be upset about that, or all that surprised........