Who said the only thing he could do is just run around spouting his chaos spiel again? There's loads of possibilities with Joker. I think if the story calls for him, he MUST be included. They shouldn't feel limited just because Heath has passed away.
And the flipside of this argument is, of course, that if the story DOESN'T call for The Joker, he shouldn't just be thrown into the mix for the hell of it. Even if Heath was still alive, this would be an issue Nolan would have to consider, but having to go through the (likely controversial) procedure of recasting the role will only serve to foreground and heighten the problem.
And I'll say it again, excluding Joker for the finale of this franchise is the same as excluding Darth Vader from the finale.
This analogy would only work if you had Star Wars be a box office success without Darth Vader, and Vader only debuted in Empire Strikes Back. Nolan has already made a perfectly good Batman film without The Joker. And I'm sure, with the wealth of Batman characters he has yet to use, he has the talent to do one more.
There is nothing that will hobble the third film faster than Nolan buying into the words of his own detractors, and believing that his film will fail if The Joker isn't in it, that it's The Joker carrying the whole film single-handedly. Nolan needs to have faith in the rest of his excellent ensemble cast, in the multitude of new characters and eager actors awaiting him, and most of all, in his brother's and his own storytelling prowess. With the level of talent on-board for this franchise, they can certainly put together a great film without The Joker.
Again, I'm not saying "absolutely no" to a Joker recast. But I'm not saying "absolutely yes" like some seem to be either. In my opinion, what the Nolans need to do is remove Heath Ledger from their minds, and go forward with their next script as if he was alive. Put together what they feel is the strongest possible continuation of Bruce Wayne's journey. Build a narrative that, like The Dark Knight, is a streamlined, focused machine. And if The Joker is an essential component of that machine once they're done, then recast him. But if adding The Joker into proceedings would come across simply as a crowd-pleasing indulgence, or a desire to simply repeat what worked before rather than meaningfully contribute in a new way, then the character should be left out of the next film.