Johnny Depp might take over Joker's role in a third Batman and honestly that's about the only person I can see doing it justice. They kind of shot themselves in the foot by killing of Two Face (although they can always bring him back) and not killing Joker (although that might have been to remniscent of the Nicholson Joker.) It's a tragedy on many levels, hopefully if they do bring in someone it is Depp. I can't really see a better person.
The only other way the could go in my opinion is have Joker do some mass suicide off camera or from far away at the beginning of the film