Jonathan Nolan is working on the script at present - "My brother is writing a script for me and we’ll wait to see how it turns out...he’s struggling to put it together into the epic story that you want it to be.”
Though he won't confirm if he will direct, the main appeal for Nolan to return is to give his creation an ending, making his work unfold in the classic film trilogy mold: "Batman Begins" (re)introducing the characters, "The Dark Knight" taking our protagonist and Gotham City to the brink, and a third one essentially resolving everything.
"Without getting into specifics, the key thing that makes the third film a great possibility for us is that we want to finish our story and in viewing it as the finishing of a story rather than infinitely blowing up the balloon and expanding the story” says Nolan who won't even hint at the villain beyond one possibility - "it won't be Mr. Freeze".
Don't expect Bruce Wayne to just continue on in Gotham either it seems, Nolan says he's "very excited about the end of the film, the conclusion, and what we’ve done with the characters... viewing it as an ending, that sets you very much on the right track about the appropriate conclusion and the essence of what tale we’re telling and it hearkens back to that priority of trying to find the reality in these fantastic stories."