Now it seems like the developer Warner Bros. Games Montréal has already started the production on another title as well. This was revealed via the LinkedIn of the studio's senior artist Megan Berry who writes that she has been working on Gotham Knights as well as a title that is currently "Unannounced". She explains:
"Co-designed features, core gameplay pillars and co-founded the project with the Creative director in addition to our current responsibilities on Gotham Knights."
The studio is reportedly dedicated to DC, so a reasonable guess would be the Superman game that was rumoured for so long, or the new Batman game there has been rumours about as well. We could also imagine that DC and Warner would like to release a The Flash game considering the fact that he is getting a blockbuster in theatres late 2022.