Even though WB Montreals previous Arkham offering, 2013s Arkham Origins, was a prequel set before the events of original developer Rocksteadys main storyline of Arkham Asylum, Arkham City, and Arkham Knight. Assume for the sake of simplification that this proposed new game is set after Arkham Knight. If that is indeed the case, then WB Montreal will have to address one little wrinkle regarding Arkham Knights ending.
Source: N4G PC Opinion: WB Montreals new Arkham game shouldnt include Batman