Saying that Robin could never feature in this series is being way too closed-minded. I mean, Robin has nearly always been a key figure in the history of Batman media. We've had other main characters like The Joker, Harvey Dent, Gordon etc. now we're probably gonna have The Riddler. When you think of other big figures in Batman from that point downwarRAB Robin is always going to feature. People just think of the tights and annoying whiny sidekick thing, but as Nolan has shown so far, there could be a different spin on it.
I think the intro of Robin would be a brilliant fun ending to the 3rd film (which surely has to have an upbeat ending). It would just be a cameo by a young actor, just one scene and not part of the story, just suggesting that's where the future could go.