After reading a few Question and Answer articles from Nolan, it seems pretty solid that the next Batman movie is going to be the conclusion, at least for Nolan.

It will probably leave off shortly after Dark Knight, where the city thinks Batman is a villain. The real villain of the movie will be someone who wins the hearts of the city but masks his true ambitions. So he with the help of the police try and find Batman's identity.

The Riddler fits perfectly. A concerned citizen by day, A serial bomber by night???