I can't believe that they would waste this much time on "Robin" if the character wasn't being setup for a spin-off series. Given the popularity of John Blake, I'm sure we'll get a Nightwing movie in a few years.
I doubt that. It would de facto be a fourth Nolanverse movie, and I think it was made perfectly clear that it ended with part three.
But the question is, if WB were to get greedy and decide to profit on nu-Robins's popularity (which could end up being short lived, it's too early to tell), would they even get JGL to play him again? Nolan's actors are known to be pretty loyal to him, no? And as far as Nolan is concerned, this thing is definitely over.