Everyone else, you've been warned.
After months of speculation as to whether John Blake would turn out to be an important character in Christopher Nolan's new film The Dark Knight Rises (speculation ranged from Robin to Azrael to a replacement Batman), the film is finally in theaters and fans now know Joseph Gordon-Levitt was actually playing in Nolan's finale. With arguably the strongest character arc of anyone in the movie, a likable actor playing him and fan interest piqued from the word go, there will likely be plenty of conversation regarding what to do with the character post-Nolan, and whether it's even right to do anything at all with him. We've got some ideas on the matter.
Let's face it: if you gave yourself a three percent margin of error and did the math, the odds of John Blake appearing as Batman in a follow-up film comes out as less than zero. Warner Brothers understands that while you might be able to get away with replacing Bruce Wayne in something like Batman Beyond, that's because part of the premise is "Hey, look at this new guy. What's that all about?" Making a standard Batman movie, the star of the show will always be Bruce because that's who everyone knows is Batman.
The only reason Nolan was able to do what he did at the end of The Dark Knight Rises is because the massive financial success of Batman Begins and The Dark Knight allowed him to have the kind of creative freedom that filmmakers rarely get on a massive, corporate franchise film.
Besides all of that, there's a simple, guiding principle at play here: the next Batman film will have to simplify the mythology in order to gel with the Justice League movie. That probably means retelling the origin story--or if we don't see a new origin story, expect it to be like Tim Burton's first Batman film--where you're dropped into a world in which the character already exists, but there's a lot of looking back over his shoulder at what put him there throughout. In any case, adding an elaborate backstory that goes back to three movies directed by someone who isn't making the new one seems like an unfair burden with which to saddle the new director, who will already be undertaking the herculean task of trying to follow in Nolan's footsteps while creating a tone that's both believable for Batman and suitable for a wider DC Cinematic Universe.
That said, expect to see Blake return as a recurring character in whatever form the next Bat-films take. Now that he's become such an integral part of one of the definitive interpretations of Batman ever to hit the mass media, it would be shocking if other filmmakers didn't want to take their shot with the character and allowed him to fade back into oblivion.
In the Comics
John Blake was not, as much as it seems like he was, an entirely original character created by Christopher Nolan for The Dark Knight Rises. A Gotham City cop by that name made a one-off appearance in one of Batman's early adventures decades ago, and so when Jonathan Nolan came to his brother with the script, it's not clear which but one of them apparently decided that this idealistic young cop should be called John Blake.
So...does that necessarily mean he'll come back? It's hard to say, but if this movie turns out to be a billion-dollar phenomenon like The Dark Knight or The Avengers, it's hard to imagine that such a popular and important character from the film not getting some love on the printed page. Renee Montoya, Harley Quinn and other fan-favorite characters have made the jump from outside media and successfully been integrated into the Batman mythology (and the DC Universe) in an important and meaningful way.
Of course, there's another element of that, which is Agent Phil Coulson. The fan-favorite character from the Iron Man, Thor and Avengers movies has now been introduced into the Marvel Universe, where he starred alongside a new, Black Nick Fury who more closely resembles the one moviegoers recognize from the summer blockbuster. Successfully integrating the character into the comics will increase his overall value as an intellectual property for Marvel, and with no real blowback from the fans, it looks as though the House of Ideas has successfully managed to pull it off. Expect DC to at least toy with the notion of doing the same.