Having said that, I'm tempering my expectations as well.....before you jump down my throat, Nolan is my favorite director. I'm sure he won't do it if he's not confident it will be good, but with the way the last movie was, it seemed to be setting up a final act between Batman and the Joker for part 3. The speech Ledger gives while hanging upside down is what I mean. He says something like "we're destined to do this forever, you and me."
Without Ledger, and without Two Face/Harvey Dent, I don't see what can be done in the third one other than include some characters I don't want to see again, such as Bat Girl,Robin, or even Penguin/Riddler.
But you may know more than me about where they're going with this. I'm just basing this off how the last movie left off, it seems like without Ledger back there's just no point in continuing the story. That movie was Batman v. Joker. It alludes strongly in the script to an upcoming Batman v. Joker II, which we'll now never see.
Where else can Nolan go with it?