It looks like the next Arkham game won't be a sequel, but a prequel.
Back at the VGAs last December, Rocksteady stirred the rumor pot for the next Batman Arkham game when Joker, during his acceptance speech, was seen tossing aside a script labeled "Arkham World." Publisher Warner Bros. was quick to dismiss the rumor as a joke.
It's time to revisit the rumor. Rocksteady is indeed not developing a sequel, in which the natural progression would be from asylum to city to world. All jokes aside, Variety is reporting that Warner Bros. has an Arkham prequel planned, set in the Silver Age of comics in the 1950's. The game is to be set right around the time that Batman first meets Joker.
The Arkham prequel is also said to include other DC superheroes from the Justice League, although no specific names were mentioned.
Until Warner Bros. or Rocksteady give official word about this. The next Arkham game will have to remain a rumor.