If there is a sequel to Dark Knight who will play the joker?

It seems like there is more to tell about the joker at the end of the Dark Knight. But I cant imagine any replacing Heath Ledger in this movie. So where do we go next? Simply ignore the fact there was ever a joker, say that he died some time between 2 & 3, or find a replacement actor? If we find a replacement, who could possibly follow in Heath's footsteps?