And the source is James Gunn himself.
As you probably are well aware of, DC is currently working on a reboot of their cinematic universe, which kicks of next year under the leadership of James Gunn. This ends the current universe and is expected to also affect video games.
So… this could mean that the upcoming Suicide Squad: Kill the Justice League will be the last game in the Arkham universe, that was once started by Rocksteady Studios with Batman: Arkham Asylum in 2009.
