According to Heroic Hollywood, Medelsohn's decision to take or pass on the role rests largely on who signs on to direct the film.
Medelsohn will be best recognized by superhero fans for his role as Daggett in The Dark Knight Rises. Since then, he's had roles in Exodus: Gods and Kings and the Netflix series Bloodline, and has a role in the upcoming Star Wars standalone picture Rogue One.
Created by Len Wein and Bernie Wrightson, Anton Arcane is the arch nemesis of Swamp Thing, rumored to be played by Ron Perlman in the film. In his original continuity, Arcane was a mad scientist who created a legion of monstrous henchman called the Un-Men. In the New 52, Arcane has a connection to the Rot, the force of death and decay in the DC Universe. He's also the father of Abigail Arcane, Alec Holland/Swamp Thing's love interest.
Justice League Dark is rumored to be going into production in 2016.