First off, it never said anything about him actually becoming the Robin crimefighter. In my opinion, that was just a reference since he was Batman's new sidekick in the film. It is also a reference to the fact that, IN MY OWN OPINION (so this may be wrong), he is becoming the new Batman. This is where I lose a lot of people, but please hear me out. First off, as Bruce Wayne/Batman has said MULTIPLE times within the trilogy, Batman is more than just a man-he's a symbol. And he even said it outright in the new film that anyone could be a hero like him (when he was talking to Gordon at the end). So even though Bruce Wayne/Batman is assumed dead by most of Gotham, he can still return in the form of John Blake, because once again, while a mere man with a cowl and cape can be defeated or destroyed, Batman cannot truly die because he is more than a more than a man-he's now a symbol and legend. My second point I like to bring to the table is that Gordon mysteriously found the Bat-signal fixed atop the MCU building. Now, this obviously means that Batman is not truly dead and that he will return. Again, I lose a lot of people though, because they immediately assume that it means Bruce Wayne's still isn't done with the cape and cowl. But I strongly disagree. That ending scene with Bruce, Selina, and Alfred at the cafe' very clearly indicates that Bruce has finally moved on and found his happy ending, and thus given Alfred his happy ending as well. In my opinion, just that secret smile between Bruce and Alfred says more than enough words. But then, who's going to fulfill the promise indicated by the fixed Bat-signal? The only logical answer is John Blake. My third point isn't quite as strong, but I think it's still just as important though. At the very last scene, Blake is elevated by the platform in the Batcave. Now, that probably means very little, but there's two things I'd like to point out. One, it's on one of those very platforms that the Batsuit rests, as shown throughout the movie. And two, I think it once again comes down to symbolism. This movie, obviously, shows how heroes rises from the darkness. And when Blake rose up in the Bat-cave, I got the impression that the Dark Knight was finally going to rise from even death, not because he's an immortal man, but because he's now the legend of Gotham City. I hope this helps, and if you have your own interpretation, I'd love to hear it!