"Reboot" is the new name for "remake" these days. Unlike "remake", "reboot" implies the studio and the producers are breathing in a new vibe and a whole new story line for a movie saga that has already been made by maintaining the original premises and signing off the previous cast and crew of the older version.
It is too obvious to fans of Christopher Nolan's Batman trilogy that the franchise has not completely ended despite the fact that the director and main lead have confirmed they will not return for a fourth installment. This franchise already has a strong fan base worldwide. It is seriously doubtful if the producers are just going to leave it there untouched with all the money they can make out of it. Remember the Star Wars Trilogy? That can happen again anytime, only with a different director and main lead.
1. Robin as The Wonder Boy
Quite unlikely since Officer John Blake's name already contains the name 'Robin.'
2. Robin as the New Batman
In the comic books, Robin became the new Batman temporarily after Wayne's alleged death. This could be a potential plot for the producers to pick up on. Seriously, how many superheroes does one city need? Suppose a new super villain suddenly appears in Gotham, or maybe The Joker had escaped from Blackgate Prison during the events of the third film and is wrecking havoc in the city. So Batman is once again needed, but Bruce Wayne has been made unavailable. Since John Blake had inherited the Bat Cave, it is only fit that he emerge as the new Batman.
3. Robin as Nightwing
After growing tired of being a sidekick, Robin renames himself Nightwing. It is uncertain how far the producers will go with this idea since there was never an actual Robin to start with. But having a whole new hero would be a great idea. Just like Batman's suit, Robin's Nightwing costume was a high-tech suit specially designed for his high-flying acrobatic style.
4. Return of The Joker and The Scarecrow
There is almost no doubt that Cillian Murphy will return to perform a cameo as The Scarecrow since he has been doing that exact some thing for the pass two sequels. But will fans be eager to see the return of The Joker? But with Health Ledger no longer with us, who could play that role as brilliantly as he did? Johnny Depp, Health's good friend seems capable of playing creepy characters like that. But it might be quite a risk since fans might not be too welcoming in seeing another actor play such an iconic character.