It turns out Rocksteady Studios is not done with Batman just yet, but whether it's another Batman Arkham game, additional Batman: Arkham Knight content or Batman in a Justice League video game is unknown.
While the co-founder of Rocksteady, Sefton Hill, previously stated Batman: Arkham Knight was the finale of the series, in the Batman: Arkham Knight DLC preview video (below), more Batman content is teased.
The video features Sefton Hill being asked if they are “categorically” finished with Batman, when the video cuts out with Hill seemingly starting to say "Well..." to an image of Scarecrow (pictured above).
Obviously we are to take that to mean they are not done with Batman just yet.
WB Games has also confirmed two DC games are in development, with one said to be Suicide Squad. The other is thought to either be a Superman or a Justice League game.