Hmmm... Well, I hope that Christopher Nolan signs back on for the sequel and the third movie. The last rumor I heard was that the next film, if Dave Goyer has his druthers, would feature The Joker as the villain with Harvey Dent as an ally to The Batman and Gordon. The Joker would be captured at the end of the film to return in the third movie at which time he'd be the one to scar Dent's face, paving the way for Two-Face (which makes sense... It's the sequel to the sequel).
As for casting, again the rumors for Dent have been consistent with Guy Pearce in the role (brilliant casting if you ask me). Others that could do a good job with the character would be Ewan McGregor or Jude Law. James Mardsen would do well, too. Essentially, any incredibly good looking, young guy with gravity is what you want.
As for The Joker, I've always thought that Jim Carey, in the hands of a strong director, could do the role justice. The Joker is supposed to be funny as well as an incredibly evil sociopath. Carrey has the build and the face for it (remember how he looked in
The Mask?). However, there were rumors that Lachy Hume was the front runner. The guy certainly looks like The Joker, although I can't for the life of me remember what how he struck me in the
Matrix films.
For more on Joker-casting rumors, check
this out.