There's no real reason to think that JGL is anyone other than Blake. Most people like to cite my ideas that Tate didn't have to be Talia as an error on my part.
Look; Tate didn't HAVE to be Talia as time went on more proof came out but at that start it was just fandom insisting that since Marion had an accent she had to be Talia which is stupid, even if it's right.
With JGL it's even MORE stupid because we have 2, count them, 2 scenes with JGL and Bale. One where he's escorting Bruce into a police car, they share no words in what we seen and they drive off. The other is in the masked ball and all we see is JGL is dancing in the background.
Other than that there's no real indication of Robin, Azrael or anyone else. He's supporting Gotham clearly but every character that supports Gotham doesn't have to be a secret character. Was Rachel actually Vikki Vale? Was Stephens actually the Flash?
Sometimes as a writer you like to write your own characters and have them do some amazing things, when adapting other works you aren't always confined to using characters that already exist.
We haven't seen much of it but from all the info we have it's a little clear that it's GORDON who takes Batman's place and starts a rebellion to fight against Bane. Blake's character description says he's on GORDON's special task force. If anything he's like Gordon's Robin if we're looking at the facts.
I've said it a bunch. Blake is an extremely important character to the story in terms of understanding Gotham's mind and Gotham taking control of itself and fighting for it's very survival. He's a character who loves his city and throws himself deep into the mix and faces danger after danger to protect his city. No different than Bruce, Rachel, Gordon, Harvey or anyone else who stood up against crime in Gotham.
The difference this time is that he's not aided by Batman, he comes in a time where it's just Mortal vs. God and THAT commitment to Gotham is a wake up call for someone like Bruce who, stymied in his self pity, sits around and initially can watch Gotham slowly come undone.
The point is that there is only ONE Batman and that's Bruce and only Bruce can bring Batman back and fight off Bane but when he doesn't do that, average people like Blake, Gordon and Foley go to EXTREMES to fight Bane. That sacrifice, that willingness to put themselves in a situation that seems like certain death is the spirit of Gotham, the inspiration of good that Bruce talked about in BB and TDK. Not Brian Douglass trying to fight crime with guns as Batman but cops going the extra distance to fight crime and in some ways average people like Selina Kyle putting their lives on the line to fight this crime.
When Blake sees those bridges collapse in front of his eyes, it's a moment. #1 this is happening to his city #2 He's starting to understand this is what HE's up against, without Batman, Just his gun and badge and people like Gordon.
And he still goes forward. This is why he's important because when there's nothing left in Gotham, they're going to need someone like Blake who will help rebuild a city broken in multiple ways.