The impression of a living Gotham City I think would be the next big step, yeah, if it were to follow the template of the Arkham games so far. It would need to draw from a pool of familiar concepts, but make some significant change or addition that impacts the overall design and presentation of the game. Much like City did for Asylum.
Obviously they're going to stick with the open world idea. Populating that world and structuring events to be more dynamic would be a pretty big change, along with main quests and side quests having deeper integration into the ebb and flow of the population interacting with the city. If it were big enough you could have scenarios like cops chasing after dudes in a van that bats must intercept from the skies.
Issue I could see is that structure would assuredly impact how much play space you have on the ground. I can't image the theme of a living city would work with bats casually walking populated streets. But that itself could be turned into a "notoriety" system, where you can walk amongst plebs, but frequency can attract certain events and negative attention towards bats.
Will be interesting to see what, if anything, Rocksteady does with Arkham 4 that sets it apart from City. I think the leap between Asylum and City, and the differences it introduced, was so great that it's going to be very hard. But it'll be a next gen game, so there's a good start. Origins following City's template is understandable, as the game has to be pushing current gen hardware limits. City size, density, population, and complexity of dynamic events has greater potential on the next round of hardware.