Even though it's been several years since The Dark Knight Rises closed out Nolan's trilogy, the ending to the film certainly left the door open to more exploration of his grim vision of Gotham City. With Blake taking over the Batcave and plenty of the comics' villains as-yet-unseen in the series, there was a litany of material to make this film series a quadrilogy or even larger.
The would-be star of such a sequel concept, Joseph Gordon-Levitt, has since cast doubt on the idea of Nolan's series ever getting a fourth installment, telling Cinema Blend, "I know we're all used to the sort of Marvel movies, which are just kind of endless series. They don't really have a beginning, middle, and end. But I think Nolan very much thought of that movie as a conclusion, and there's a theme that runs through all three of those movies that begins in the first movie, runs through the second movie and it concludes in that moment where he says that Batman is more than a man, Batman is a symbol. And so to have another man other than Bruce Wayne kind of becoming Batman at the end of that trilogy, I think that's the perfect ending to that story." However, the film itself didn't foreclose such a possibility, so never say never — even if Nolan and Gordon-Levitt aren't interested in returning to the material. (Bryan Singer's Superman Returns was a sequel to Superman II, after all, so another filmmaker could, in theory, try to pick up where Nolan's series left off somewhere down the line.)
On the other hand, if The Dark Knight Rises does prove to be the end-all be-all of The Dark Knight Trilogy, that also helps to establish a standard that not every superhero series has to end on a conclusive note. Rather, this film opened a door and gave us all a peek at what could be with the futures of Bruce Wayne and Blake and left the rest to our imaginations.