I still The Dark Knight's (and Christan Bale's Batman's) best villain died with Heath Ledger.
That's the thing.
You know, The Joker probably had a Death sequence in the movie, which was cut out due to his actual death...
So in that regard, I wouldn't expect the Joker to be a returning Villian. Most villians are killed in the films these days, the Sandman in Spiderman being the exception.
I don't think they planned on killing the Joker. That would be a stupid, rather cliche move and I don't think Christopher Nolan would kill off the most major Batman villain after one movie. I just can't see it happening.
I heard they did film the Joker's death, then left it out due to "respect for the family" after Ledger died.