I think that Two-Face has to be prominent in the third movie. I don't there is anyway around it.
What bothers me, is where now? The Dark Knight used two of the most iconic characters in the franchise. How do you top that for a third?
The Joker can't come back, and not just because of the death of Ledger. His story arc was used and worked wonderfully. We don't want too much of a good thing.