The ending to Arkham Knight is intentionally vague, and theories abound.
With Batman's identity revealed, Bruce Wayne feels like he needs to take some drastic action to prevent the inevitable fallout. Does he kill himself and Alfred? Given that Batman is staunchly anti-murder, I can't believe that's the case. More than likely he's faked his death in order to go into hiding, much like the end of Nolan's The Dark Knight Rises. Plus, we've already had several death fakes in the Arkham series, so what's one more?
From the post-game cutscene you mention, Gotham still has a bat-themed protector. It could still be Bruce Wayne, reprising his role as Batman despite his "death." This could be Azrael, who is training to be Batman in several of Arkham Knight's challenges. It could also be some other person - Robin, Nightwing, etc.
Until Rocksteady or Warner Bros. decides what's next for the series, we won't know. I find it 100% likely that there will be more Batman games in this vein at some point in the future...