8. Co-op Multiplayer
After Arkham City marked the natural progression of the series by introducing Batman, Robin, Catwoman, Nightwing as playable characters, it seems very clear where Rocksteady - or whoever actually makes the next game - should go, and that's towards a full-on co-operative multiplayer mode. Though recent games like Dead Space 3 have been divisive in how they appended a co-op mode to the main campaign, as long as it's not intrusive to the single-player experience, it could be absolutely fantastic. Imagine playing as Batman while your mate follows as Robin or Catwoman; together you're an immense crime-fighting duo and can even complete co-op challenges together. If this was achieved both online and locally, that would be the icing on the cake too, given how many developers now shun offline play altogether, essentially putting all their eggs in one basket.