There has been a lot of speculation about what's next for both Rocksteady and Batman games since the developer completed its Arkham trilogy with Arkham Knight last Summer. I think I might have a few answers for you now after a little prodding.
I've been doing some digging for quite a while to find this information after I caught two whiffs of a Superman or Justice League game coming. The first was this gif said to be from a Superman game in the works by Arkham Origins developer WB Montreal. This animation of Superman has never been confirmed or discredited, but it always 'felt' right to me. Hardly concrete evidence, but something to consider. The second clue that started me on the track was an Easter Egg in the Batgirl dlc for Arkham Knight. In there, there was a secret room showing the Justice League's first ever villain Starro in a tube. This DLC in particular was made by WB Montreal, which is important, so keep that in mind.
Now I have been privy to multiple sources that say Rocksteady indeed are not working on a new Batman game, and it was heavily implied to me they were not working on anything DC related. I've heard phrases like the studio 'is moving in a totally different direction' more than once. My best guess is that the studio will be launching a new IP, but that is pure conjecture.
This does leave a door open for WB Montreal and it's already been confirmed that the studios is currently working on two DC Comics games. Word on the street for years has been that the studio is working on a Suicide Squad game, but that has been a very quiet of late for a long gestating rumour. In all my prodding, reference to that hasn't come up, so you can take that as you will.
I have however heard more than a few subtle hints that their next games will include Superman. What I haven't identified is in what capacity. It's unclear if this is a standalone game, a Superman and Batman game or a Justice League game. I don't concretely know right now. However, my educated guess is that it's leaning more toward the Justice League. This is based on the Starro easter egg and the now ramping up movie franchise making it timely. On top of that, Nolan North recently said he signed up for a new franchise that will be "another 10 years of his life" at WB Games. North played the Green Lantern in Justice League: Crisis on Two Earths as well as Superman in Young Justice too. (He could just be working on another WB property entirely too though.)
While a Batman/Superman game could make sense too, I feel that would come at least in the same year as the film. With no announcement thus far, I just don't see it happening.
And that is what I have. I realise none of this 100% confirmed, but from everything I have been given, this is the picture I've been able to piece together. You can feel free to make your own conclusions though.