1. The Dark Knight Returns
Since Nolan has left Bruce Wayne alive in his film world, it's not too beyond the pale to think that a future film could come along that brings the character out of retirement. Bale has gone on record saying that he isn't interested in playing the role again without Nolan in the directors chair, but he doesn't have to. If a follow up is set decades later an older actor could take on the role. Someone like Ron Pearlman or Jason Statham for example could have some make applied so as to look like a guy that's older enough than Bale that our disbelief could reasonably be suspended. There were already rumors flying around that Nolan's third outing might borrow from that play book and a lot of fans were hoping it'd be true, so there's already an audience willing to watch a movie that isn't an exact replica of the 1986 mini-series. As to what might have happened in the 30 or 40 years that Bruce has been out of the game, lots of things could have gone down. After 5 months of anarchy, the people of Gotham City might not be as interested in returning to authority as they seemed at the end of Rises. Citizens might have been more inclined to go along with the cops and the Batman just to get rid of Bane, but maybe they liked living in chaos enough that the restored peace doesn't last. There's also been a precedent set in the last several years of a new class of criminal that take things to a bigger level. Just because Batman stopped 4 or 5 or 6 of em, doesn't mean that there won't be new ones emerging, especially in a world that thinks Batman is dead. If Robin John Blake isn't up to the job, he could end up dying in the line of duty and Bruce might not be up for returning to the fray, especially if he and Selina have a family by then. If enough decades go by with no protector, Gotham could turn into the hellhole that it was at the start of Frank Millers classic series. It could be at Selina's funeral, with his fully grown and adult children standing by that Bruce decides that it's time to go back to the one thing he's ever been great at. An elderly man walking around in a set of techno enhanced armor doesn't seem so silly now that we've seen Bruce using the leg enhancement that helped him walk around in Rises. Really the only thing that would feel wonky about a 4th Bat movie picking up so far in the future would be the jump over what did happen. Then again, such a film doesn't have to be the very next one...