Rocksteady Studios' next game is the subject of a vast amount of speculation. Rocksteady's most recent game Arkham Knight is nearing its fifth anniversary and no game has been announced since, despite rumors that Rocksteady was preparing an announcement for some time now. While there's no news yet on that front, a new leak does hint at what Rocksteady may have wanted to work on following Arkham Knight's launch -- a Superman game.
Leaker James Sigfield claims that they have multiple anonymous sources that have verified the news to them. "Rocksteady pitched a Superman game to WB as a follow up to Arkham Knight, and WB passed on the project," is what Sigfield claims. He doesn't add any further context for the pitch or Warner Bros.' rejection.
What Sigfield does add is a link to a rumored leaked piece of Rockstead concept art showing superman flying around a futuristic Metropolis. "Judging how similar this feels to the 'New Gotham' section of Arkham Knight, this could be for that," Sigfield speculates. As in, the piece of concept art could have been a part of Rocksteady's pitch to WB for a Superman game.
As far as rumormongers go, Sigfield reliability is certainly worth questioning. It would be easy to say the rumor was built off of the leaked piece of concept art, considering it didn't pop up any sooner. And there have been a lot of Rocksteady-related leaks and rumors over the years. Without further evidence to support this rumor, consider it highly questionable.
What might add some context to the rumor would be if Rocksteady officially announced its game. Recent rumors hint that Rocksteady is working on a DC Comics game that isn't Batman focused, but rather covers a broad range of DC superheroes. Superman could easily be one of those characters and this piece of concept art could easily be that. Rocksteady may never have made a Superman pitch, but rather has always been focused on this new project.
If WB did reject a Superman game from Rocksteady, it would be somewhat understandable. Superman as a hero isn't anywhere near as popular as he used to be. Superman comics don't sell considerably well, but more importantly, there hasn't been a pure Superman movie in a long time. Even Superman's most recent movie wasn't named Superman, but rather Man of Steel. Where a Batman game would get approved instantly just based on marketability, Superman may struggle. Superman as a member of the Justice League, however, may go over much better.
Rocksteady's current game in development remains unannounced.