After (yet) another viewing of TDK, I'm beginning to think we may see several villains. Considering the theme of escalation and a few hints, like the reappearance of Scarecrow, the whole fear gas in the Narrows thing, and Joker's comment at the end that "this city will be doubling up the rate which people are losing their minds", I wouldn't be surprised if a couple of things happen in the next movie. Like, Gordon's wife could leave him, taking the kids, explicitly because Gordon won't stop working with Batman. This puts even more stress on his relationship with Batman, who's having to fight the police - I could see the main character arc being all about Batman and Gordon's relationship. It falling apart and them having to find a way to work together when things continue getting worse. Nolan has mentioned on several occasions that The Long Halloween is a major influence - so by the end of the third movie, Gotham's transformation is complete, psychos have taken over all the crime, and the only ray of hope is that Batman and Gordon heal their relationship to go on to fight them. That'd be in keeping with the tone of the previous movies and it prevents anyone from having to try to equal up to the Joker as a villain.