I think it would be reasonable for Two Face to come back in as a more traditional version of the character, multiple personalities fully developed. I can't imagine them doing that in the very next movie though... which makes me kind of sad because I want this movie series to continue for a very long time, but it probably wont. The current cast (who's left, anyway) said, from what I hear, they're only willing to do one more.

ETA: Also what Roto said.