I just don't see how they can make a third. The Joker was beautifully contradictory. He was compelled to stand against batman because he could but in the end he had to validate himself. (I don't think you can ponder the philosophy of super heroes better than this script did). And, Ledger played it for all he could. You cannot top that. Also, these films have kept the overarching conflict very cerebral so far and so that leaves you a handful of choices because they have to match wits with Wayne and at the same time teach him something about Batman. Return of Raz via lazerus pit or Freeze, neither of which would seem to work in Noland's world. Maybe, The Dark Knight Returns would work. He would just have to leap ahead thirty years which doesn't seem like it would work either.