Does it have to be a man? Could it be a woman? Talia Al Ghul was killed by the Joker in Arkham City and may stay dead. However, both she and her father Ra's Al Ghul had access to Lazarus Pits to heal themselves from their injuries. From what we saw of Talia in Arkham City, she was deeply in love with Bruce Wayne and wanted him to become the next leader of the League of Assassins. Despite death and undying love, she could still prove to be the Arkham Knight because she felt let down by Bruce's actions in the previous game. Although like Ra's Al Ghul, this may be going over old ground from both Arkham City as well as The Dark Knight Rises. Talia is a highly trained assassin and does have the means to become the Arkham Knight. It would certainly be a great reveal to have the antagonist of the game be Bruce Wayne's love interest.