My thing is, if you do Nightwing, you buy everyone time. You start off with him. Eventually you get into the Azrael story and him becoming a corrupt Batman, and Wayne hears that the Batman legacy is tarnished, so he has no choice but to return. It works so perfectly it's Blue.
They hinted at Riddler in The Dark Knight, and it'd be perfect for him to start with Nightwing like he has in every Batman movie/cartoon/comic/game, and it would make the Azrael Batman even more meaningful because Riddler's intent is to expose Batman to the world. Well, you say it's Wayne and find out it's someone completely different, the mission gets deeper.
They've put Scarecrow on camera so many times it's ridiculous. You finally break through with him becoming a Godfather of Gotham crime and he's a TRUE foe for Nightwing, and could also help facilitate the return of Wayne.
There are just so many possibilities left, and with the way they ended the movie, I can't picture they avoid working on all of this in the future. Makes no sense to leave it behind where it's at.
And the only thing anyone clings to as a quote indicating that Nolan is done is, "The trilogy is over." No shit... you can't have a 4+ movie trilogy. The trilogy had no choice but to be over when it started. But that doesn't mean it can't expand beyond a trilogy.
BTW, they copped-out with the Bane/Thalia story as well. If you don't see the potential to do a Bane Beginnings movie in the future, or at least a dope spin-off comic series...
All TDKR did, was open doors. It closed a couple, but it opened way more than you'd ever think to imagine they wanted to if they were finished...