The PC version of Batman: Arkham Knight is currently being repaired and Warner Bros. as gone so far as to pull the game from Steam until those repairs are made. The performance issues are high for the game to even be sold, and that’s rough.
This hasn’t stopped the modding scene from making something really cool however. A new mod has been made available (not official) that will allow freeroam mode with Red Hood, Harley Quinn, Nightwing, Robin, Azrael, Bruce Wayne, Catwoman, Commissioner Gordon, Joker and GCPD Officer Owens. Each character can be seen being used in freeroam mode in the video above.
Keep in mind some characters aren’t completely functional in the game based on what animations where created for them. For example characters like Harley Quinn and Nightwing are fully usable because assets are in place for their combat animations. On the other hand the Joker can’t even fight at all and some characters can fight but not counter. You take the good with the bad really.
Installation instructions can be found here. Something to do until Rocksteady gets the main game fixed enough to finish.