According to a since removed IMDB listing, Tim Daly, the voice actor who voiced Superman in the late 90s animated series, is set to reprise his role in Batman: Arkham Knight.
Daly is no longer mentioned on the game's page, and while he was listed as playing Superman, there were no additional details of what to expect.
Of course, the problem with IMDB listings is that they can be altered by anyone and there's a very decent chance that this was a fake. Even so, we know Rocksteady have said that Arkham Knight will be their last game in the Arkham series and the appearance of Superman (if he actually is in the game), could be a bridge to a future Justice League game.
If there is a Justice League game in the works, it would obviously open the door to heroes such as Wonder Woman, Aquaman, Flash, and Green Lantern.
Batman: Arkham Knight launches in June, 2015 and it was recently confirmed that the game will not feature co-op play.
Whiie it's best not to take this too seriously, would you be interested in a Justice League game from Rocksteady?