After you finish the game, you can return to the museum, and in the large area with the water, if you go to the right area, you can hear three of Two-Faces thugs having a coversation that goes down sort of like this:
"Inmate 1: So what now?
Inmate 2: I say we hang here until someone tells us whats happening with Arkham City and then we decide.
Inmate 3: You think? What the hell can they do next? Arkham County? Arkham Country? Big-@@@ Arkham World?
Inmate 2: Like that will ever happen.
Inmate 1: I dunno man. These people are crazy, aren't they?
Inmate 2: Yes they are."
I lol'd when I first heard it, but it does make you wonder, where will Rocksteady go next?