I think Daniel Day-Lewis would be a good replacement for Heath Ledger, better than Joseph Gordon-Levitt. Day-Lewis is a method actor, he's known for his devotion to his roles.
But I don't think the Joker should be in the next one.
Heh, they were the two people I've been considering the most. I think Day-Lewis is probably the only person who'd even be
accepted as a replacement for Heath given how big a deal his whole story here is now, but I think Gordon-Levitt might be a more seamless change. Then again, Day-Lewis could probably emulate Ledger's performance perfectly(and I wouldn't be surprised if he actually did it
better than Ledger, for that matter), so I dunno. If I had to choose, I'd rather see Day-Lewis play the role. Whether or not he'd actually do it is another story, because he might think he was somehow insulting Ledger by doing it.
In terms of the whole "shouldn't appear again without Ledger/no need for him to come back" issue, I completely disagree that he's done here. The role was never intended to only be in one movie. They specifically made it a blue-balls, battle-but-not-the-war scenario because they wanted to bring him back in the next one. An actor's personal life(or, I guess, death) should NEVER interfere with planned, crafted storytelling(one of the reasons ongoing TV series can still be seen as a flawed medium i.e.
LOST with whole storylines scrapped and characters bizarrely written out because the actors want to do something else). If they want to do the Joker, do the Joker. I think Heath Ledger would be the first to say that he'd want the character to come before himself.
By the way, was anyone else disappointed by the lack of the Batcave and Wayne Manor? That's the only thing that stops this from being as close to a definitive Batman movie as you could make today.