According to Variety the sequel to Batman Arkham City will be actually be a prequel set in the 1950’s, and will be ‘highly stylized’.
The game is said to revolve around Batman’s first meeting with the Joker and will be based on the Silver Age of DC Comics from the same time period.
This is the time in which Batman and chums founded the Justice League of America. I expect Kris is running about the room screaming like a girl right at this moment.
Variety say the game is most likely to appear in 2014.