Have you ever notice a villain called "Arkham Knight". from my point of view he's the main villain of this game because of the title also arkham knight and also the villain's name too. you can check left side image in this post. He's look like same as batman but he's very different and super duper devilish villain. It's pretty amazing to see the villain who had already surpassed batman. :)
But there is a mystery behind Arkham Knight. he could get a major role for this game. He could be "Hush" because of 2011's Batman arkham city game Identity thief mission.