I think it's pretty interesting that apparently the one thing Nolan knew about how he wanted to finish the story was bringing back the LOS, before they had even had decided on a villain.
“Moving on to Dark Knight Rises, I knew that the League of Shadows had to come back,” he says of the secret society of assassins led by Liam Neeson’s Ra’s Al Ghul, the violent Darwinist who recruited Bruce Wayne and taught him the tricks of the stealth-warrior trade in the first film. “I knew that we had to return to Batman Begins and those philosophical ideas of Ra’s Al Ghul, those challenges -- that all had to come back.
This seems to lend more credibility to the rumors that Nolan took a hard stance against Justice League: Mortal
partially because it featured Talia coming back to avenge her father, and he wanted to be able to revisit that storyline if he made a third film. Pure speculation, but it's interesting. If there's anything out there that could prove TDKR wasn't some alternative movie that they had to think up because they couldn't use The Joker, confirmation that Nolan had the idea of bringing back the LOS all along could be it.