Batman Arkham Knight launched in 2015 and since then, we have yet to hear about the new project from Rocksteady Studios. They have been reportedly working on a game but with no reveal in sight.
Currently, a new rumor states that their next project is close to getting revealed soon. The source of this rumor is a freelancer journalist with ties inside the games industry. According to him, depending on if certain development milestones are meet, Warner Bros. Interactive might finally reveal the next project by Rocksteady Studios either in Q1 2019 or at E3 2019.
Rocksteady Studios’ next project has been rumored for a long time now. Earlier rumors pointed towards them working on a Superman video game, but they later turned out to be false. Currently, there is no concrete piece of evidence that suggests anything for their next project, but it is likely going to be related to the DC Universe in some way.
Rocksteady Studios are also hiring for new staff that suggests that their new project might have some component of Games-as-a-Service model, but it is too early to call anything final on it. If the game is revealed next year, then we will finally get to see what they have been working on in secrecy so far.