Nolan, from the start (I've seen in interviews) was told, and agreed, to the notion that Batman, as a comic book 'franchise' needed to be open-ended to some extent; in fact instead of trying to interpret what he said in my own way, much simpler and better for me to show the quote, which is as follows:
And besides I thought the ending was superb, common to all Nolan films: the full magnitude of the preceeding events can only be appreciated after the film as finished and, as such, they reward repeated viewings. I saw Rises for a second time 10 days after the first and it was even better, I kid you not. He is a wonderful film-maker, a craftsman with a vision, and to doubt him is to embarrass yourself, as I have done several times before. He always knows where he is going and puirposefully overloads his films with plot, expounded through subtle foreshadowing and callbacks that require maximum concentration to become fully aware of. I know this may be an overused compliment but he really is a genius. Not many film-makers have simply floored me because of the complexity and billiance of their stories and how they unfold. He regularly has.
A (along with his brother Johnathon and fellow writer David Goyer) true talent.