After Rocksteady Studios finished making Batman: Arkham Knight they said it would be their last Batman game. Some people think that their next game is going to be a Suicide Squad or Superman game. I have even heard rumors of them making a Green Arrow game or some other DC character. They even made references to some of these characters in Arkham Knight.
Of these options a Green Arrow game would be the easiest, because they could just use the same combat from the Batman Arkham games and modify it. A superman game would be the most difficult because superman cannot get hurt by normal means. This would make the game difficult to create. People are saying that a Superman game is impossible to make, but they also said that before Arkham Asylum. So if anyone can make a Superman game its Rocksteady Studios.
I would be willing to play any DC related game made by Rocksteady studios, since I enjoyed the Arkham series so much.