I think if Nolan had planned to do DKR next he would have set the second movie a few years after the first one, when The Bat-Man is an established crimefighter that has been fighting the Joker and other bat-rogues for a while. DK, I think, still counts as a movie about Bat's beginnings. To jump from that to the end of his career would be a little abrupt.

Now, he might still do DKR next if he thinks it's a good idea, but I don't think he planned that from the beginning.