I REALLY want The Joker to return in the third film. But the more I think about it, the more I realise that I want this for selfish reasons.
The Joker is my favourite comic book character, so of course I want to see him in another movie. But when I think about it objectively, I fear that recasting Joker just to bring him back - and then trying to twist things around to fit him into the third film's narrative - could bring into action the law of diminishing returns. If this Joker is not going to match Heath Ledger's portrayal, isn't it better to let The Joker's role in "The Dark Knight" stand on its own merits?
It's not even an issue of "Oh no Heath Ledger's dead recasting him is sacrilege!" If his story wasn't complete, and they NEEDED a new Joker, I'd absolutely support it. But if they're just bringing him back because he's popular....eh, it's risky. Hannibal Lecter was great in "Silence of the Lambs". "Hannibal", not so much. Darth Vader was great in the original "Star Wars" trilogy. The prequels, not so much. Jack Sparrow was great in "Pirates of the Caribbean". The sequels, not so much. Maybe it's best if The Joker doesn't join that list?
Also, in terms of the franchise in general, perhaps its better if The Joker isn't around. It seems like The Joker's entrance in "The Dark Knight" will make that film a radically different beast from "Batman Begins". Similarly, introducing a new antagonist could help brand the third film with its own individual identity. And then we get the sense of each film in the trilogy being excellent on stand-alone merits, rather than 3 being too close a follow-up from 2.