While Rocksteady is working on an as of yet unannounced title (not Superman as previously rumoured), and they're not working on the next game in the Arkham series. However, Warner Bros. Montreal seems to be hinting at an announcement too - so what is exactly happening with the Arkhamverse? According to one central character, it will continue.
Recently a fan stated that he believes that Bruce Wayne's time as the caped crusader is done within that universe, and here, surprisingly voice actor Kevin Conroy, who has voiced the character for many years, stated the following:
"I don't know....I can think of ways to have Bats still evolve in the Arkham world"
While not serving as a confirmation of anything, it is a change of tune since last year, when Conroy openly stated that there were no further plans for Arkham games. Back then he only mentioned Rocksteady Studios, so perhaps it stands to reason that a new Arkham is underway but from Warner Montreal (Batman: Arkham Origins).