The Dark Knight producer Charles Roven was asked by MTV whether they would’ve done anything different with Ledger’s Joker if they knew the actor wouldn’t be reprising the role:
“We have to separate the actors from the role,” said Roven.
“On a personal level, Heath was a friend of mine. We had worked together before ‘The Dark Knight,’ but I still think that ‘The Dark Knight’ is its own thing, and we have to separate them,” added Roven.
That certainly seems to indicate that they’re willing to recast the role, should the story call for the character to return.