With all the playable characters there are for Arkham City - Batman, Robin, Catwoman, Nightwing - Rocksteady might as well put co-op play in the next game.
It doesn't have to be shoe-horned in to the main single-player campaign. In fact, we'd rather it wasn't, but a separate mini-campaign would be brilliant. Two-player Riddler puzzles would work nicely as well. Kind of like a darker, deadly version of Portal 2.
Co-op Predator challenges could be fantastic too, having one player strike the enemy while the second distracts. The Free-flow combat system could be expanded with double-team counters, takedowns and strikes too.
And, if we're not asking too much here, free-roaming co-op in an open world Gotham would cause us to collapse in spasm. After all, what's the point in having all of Batman's side kicks in the game if they never get to team up with each other?