and let him go off and do this thing. No film maker wants to work on someone elses things forever. Especially a true artist like Nolan.
It should be 'closed' in a sense that we feel fullfilled with this trilogy, but the door should be open for another one.
No DIRECT clues to the next villain, just that another one has arrived. All we need is the batsignal, which every one seems to be in agreement about that it will be the closing moment of the film in one way or another.
No evidence. No winks to the camera. Just the signal. It's a sign that Batman is needed and there's trouble. Which leaves us wanting more. And Nolan shouldn't be like "they should have ______ as the villain in the next one" and screw the new director. As an artist wanting to have his own take on things, I think he'll respect the next artist to take over the franchise.
In a metaphor: Nolan will close the door behind him......but it'll be unlocked and the porchlight will be on.