I don't intend to get much into the Dent is dead/alive debate. There is a thread that is revolving in an endless circle for that already.
But I will say that it is worth noting that Nolan doesn't like to repeat himself. He cut to the meat and deepest stuff of Two-Face's character and motivation in the Dark Knight. He needs convincing to do this one at a time and that is why a third movie isn't even set in stone. I imagine if he does one he'll want to it to at least be something new, fresh and original, even if he cannot top TDK.
That is another reason why I don't think we'll see Two-Face again. He wants to move on.
With that said Eckhart was simply amazing in this movie. I agree he (and Oldman and Bale as well) are being completely overlooked because of Ledger's amazing turn. While I think at this point Ledger DESERVES an Oscar for what he did with JOker, Echart's Dent should be nominated. Though there is no chance of that. But Dent is the emotional centerpiece of this movie and his arc from hero to villain and from Batman's idol-almost to needed martyr is all very fascinating. Echart played the part so well there is an enormous sense of sadness seeing what he is reduced to and still at the end when he is holding a gun to a child's head much of the sympathy has faded in way of a truly pathetic and selfish villain. Yet his "death," when Batman turns his head to the good side is still very moving, poignant and the symbolic point of the movie. That is the main reason I don't want to see Two-Face back. It was so wel ldone and I think in those final five minutes said everything that needed to be said.
With that said, sorry, I'd like to see a cameo for Joker in the third movie. Just Batman going to Arkham some reason (maybe seek advice reluctantly from Dr. Crane) and on the way to his room he passes The Joker's, which is marked unnamed or unknown. When he passes we get a shot from the inside of a figure with green hair perched in a shadowy corner giggling "Why so serious" to Batman who ignores him as walks by.
Just my little idea.