I've yet to play any Batman: Arkham– games. Might pick this one up in a couple years when the price drops. I'll get started on the ones I do own (Asylum and City) at some undetermined point in the future.
I do know there is a mod for Arkham Knight that allows you to play as any of various characters in Free Roam.
Video link that CONTAINS SPOILERS. Spoilers as in minor stuff like "oh, you can play as X" AND major stuff as in "oh, you see Y".