Think about the Arkham series: Asylum was phenomenal, City was solid and Knight, well, it still played fantastically, but the story was all over the place.
Overall though, Rocksteady seemed to take the franchise in the direction of GTA - or that of an open-world template - and they really didn't need to. You wouldn't even have gotten the naff 'tank battles' and forced Riddler Raceway challenges in Arkham Knight, if not for Arkham City's push towards a bigger, more open game world.
Even considering how Arkham City is laid out, it's a series of enclosed spaces and levels, peppered across an expanse of Gotham that is barely a city at all. There are no pedestrians, no signs of life other than gang members to slap around or the occasional 'Political Prisoner' in need of rescuing - I mean, why was Arkham City set in an open-world in the first place?
It would've been far tighter and better-plotted as a more linear experience with the occasional large area, a la Arkham Asylum, or more recently, Uncharted 4. The more you think about it, the less sense it makes.