I'm not sure exactly where this is, but Nolan stated somewhere that he was interested in ending TDK in a way similar to BB, in which it excited audiences about the possibility for the next chapter.
Now perhaps all that he intends is for Two-Face to emerge at the end of TDK, which would be a great twist for people who aren't familiar with Dent's tragic storyline. But there were two other rumors: one was that Bruce would get a call from Selina Kyle looking for a date (or something like that), and the other was getting a card with a "?" printed on it (implying the Riddler.)
Personally, I see no problem with introducing Selina Kyle. Many fans don't like Catwoman as a villain, but she's not really exclusively a villain - she's more of an ambiguous character who is on her OWN side, sometimes siding with Bruce and sometimes siding with the freaks. I think that could be an interesting dynamic, and it would create a buzz for the next film in regards to what actress could fill the role. If Nolan wasn't afraid to re-imagine the Joker, he certainly won't be afraid to re-imagine Catwoman.