I'm very disappointed to hear that the Joker is being written out, if it is true. I was actually warming up to the idea of a recast. I mean, yes, Ledger is going to turn out to be brilliant, there's no doubt about that. But the Joker as a character has existed for 68 years, and has been played by various actors. I guess what I'm getting at is that the character sort of transcends actors, and should not be limited to just one "definitive performance".
I don't want to see the Joker become an untouchable role because of Ledger. The man was a professional, not to mention a fan of the character, and wouldn't have wanted the buck to stop there.
Mind you, a recast would be walking a fine line. The actor cast cannot simply do a Heath Ledger impression, because that, above anything else, would be disrespectful. Something new should be brought to the role, because, anyway, it would be boring otherwise.
However, there should be some level of continuity, to where you can believe you're watching the same character. Which means the actor must be at least somewhat similar to Ledger in appearance, and have the acting chops to both do something different and keep in tact what Ledger did.
If they can do that, a recast could work, and work well. It would be almost symbolic, that the character transcends actors, in much the same way that James Bond (or perhaps a more analagous comparison, Ernst Stavro Blofeld) does.
Although, the actor would have to know that it would be a thankless role. Unless Ledger wins an Oscar or something, I doubt anyone would be likely to praise the new actor as good or better than Ledger. My point being, that if Ledger isn't properly recognized, the new actor never could be. It would be hard to live up to what Ledger created, and to try and out-do him would be an insult. Which is why I think the actor has to be good enough to fulfill the qualities above, but also understand that he's not trying to be better than Ledger.
The actor should be able to rival Ledger, not try to out-do him. Simply, the actor just needs to do both the character and Ledger justice.
Also, if they bring the character back, they really need to do something with him. Don't just have him be there for the sake of being there. Give the actor something to do, so they're not just being cast to replace the dead guy (think about what Ed Wood did in Plan 9 from Outer Space. He basically hired someone who looked vaguely like Bela Lugosi, stuck him in a cape that obscured his face, and gave him no lines. That's what some people have suggested they do with the Joker, and it's exactly what they shouldn't do.)