I don't even know if they're making another Arkham game, but I feel
like they are.
Based on very random rumors and Kevin Conroy letting slip a while back that he was working on a new Arkham game (NOT Arkham Origins), I just get the feeling that Rocksteady is secretly making a new game for XBoxONE and PS4.
It always seemed to be that the reason Origins was made by a different company was because it was being used a placeholder until the arrival of the new "major" Arkham game. It was basically something to tide us over while Rocksteady worked on their next installment.
Personally, I think there absolutely needs to be another game. Arkham City was fantastic, and parts of its story were clearly laying the groundwork for more to come.