3. Batman and Robin
Tying in with the idea of introducing multi-player co-op, I think it's about time that Rocksteady started to properly develop the bond between Batman and Robin as a crime-fighting duo, and do away with those nasty memories we all still have of Joel Schumacher's dire cinematic take on the team. A step in the right direction was made by actually introducing Robin in Arkham City, complete with a great new look that hewed far away from the campy "Boy Wonder" aspects of the character. For the relatively brief amount of time you can play as Robin, he was an absolute blast, so if the co-op campaign was going to focus on any character in particular, it should be him. Devote cut-scenes to developing his dynamic with Batman, and really accentuate the bond that has largely been missing from this property in all its forms for quite some time. Given how complimentary the two characters are, multiplayer combat that puts their various strengths and weaknesses into perspective could be stellar.