"You won't kill me because of some misguided sense of self-righteousness. And I won't kill you because you're just too much fun! I think you and I are destined to do this for a long time."
That kind of foreshadowing practically DEMANDS follow-up. I don't see how the Joker can possibly NOT be a major character in the next film.
I say re-cast. The great thing about the Joker is that he's chaos and destruction incarnate; there's no actual need for the next actor to play him the same way that Ledger did, because the character is an inherently chaotic one whose behavior may shift from day to day. (Goodness knows his own accounts of his past do.)