Christopher Nolan Explains the DARK KNIGHT RISES Open-Ending which really didn’t need to be explain

The Dark Knight Rises is specifically and definitely the end
of the Batman story as I wanted to tell it, and the open-ended nature of the film is simply a very important thematic idea that we
wanted to get into the movie, which is that Batman is a symbol.
He can be anybody, and that was very important to us. Not every
Batman fan will necessarily agree with that interpretation of the
philosophy of the character, but for me it all comes back to the
scene between Bruce Wayne and Alfred in the private jet in Batman Begins, where the only way that I could find to make a credible characterization of a guy transforming himself into Batman is if it was as a necessary symbol,and he saw himself as a catalyst for change and therefore it was a temporary process, maybea five-year plan that would be enforced for symbolically encouraging the good of Gotham to take back their city.To me, for that mission to succeed, it has to end, so this is the ending for me,and as I say, the open-ended elements are all to do with the thematic idea that Batman was not important as a man,he’s more than that. He’s asymbol, and the symbol lives on.

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