3. Superman
Before you dismiss this idea, please hear me out. I do NOT want to see Batman and Superman team up in this world or play DLC as Superman as I can't see anyway for that to be realistic. I mean come on! Superman could just lift Arkham City up over his head and shake out all the bad guys into the river. BUT what if Superman were a villain? Ever since Frank Miller's The Dark Knight Returns, the world has had a fascination with Batman fighting Superman. Also in the comic series Hush, Superman is brainwashed and used for evil as a bruiser against Batman. Batman of course never loses. So that being said, a brutal fight between The Man of Steel and The Dark Knight would be amazing for a gamer to play through. Think Batman spraying some Kryptonite gel on his knuckles and getting down to business. Or an aerial fight with Batman getting towed around the skyline by a grappling hook attached to Superman's leg. If this isn't in the game, it would be a huge missed opportunity.