6. Vehicles
It blows my mind that this hasn't already been included in the Arkham franchise to date, but then, it is quite an undertaking, as we can all remember games where driving mechanics were completely out of step with the rest of the title, and resulted in some very good games becoming slightly less good. Still, it's an indelible element of Batman's repertoire, and one that the series has to surely confront eventually. It's easy to respect Rocksteady's devotion to what they know, but the combat and exploratory elements have been so well-honed that I think it's about time they challenged themselves yet again. As long as they allocate adequate resources to it, I have absolute, unwavering faith they won't screw it up, assuming a wide enough location is selected to roam around in, and a mechanic is introduced to allow pedestrians to flee out of your way at the last minute, of course. Also, perhaps it's expecting too much, but what about including the Batwing? Update it to the rough specs of Christopher Nolan's "The Bat" from The Dark Knight Rises, and that could be a stupid amount of fun.