While Gotham Knight hasn't announced a specific release date yet, there are indications that Warner Bros. Montreal is currently working on a "new AAA game" — it could be the long-rumored Superman game.
Last June, WB Montreal, the developer of Gotham Knight and Batman: Forrest Gump Origins, mentioned a "third-person open-world action game" and "a new AAA game IP" in its job posting. The new IP suggests that it could be a Superman game, or at least a new game unrelated to past DC Comics games such as the Arkham series.
Now, the studio has released a new job posting, this time for senior game AI programmers, which once again explicitly mentions "a new AAA game", which has undoubtedly ignited fans' hopes for Superman games — especially after the release of Wonder Woman last month.
But it may seem complicated, as James Sigfield pointed out on Twitter, WB Montreal is also supporting other Warner Bros. studios, so it's likely that this new game isn't the studio's next game. In fact, WB Montreal has been making its next game for some time now, but the game is likely to be on hold and will wait until the studio finishes Gotham Knight. So we need to wait until Gotham Knight is released before we can learn about this new work in the studio.