If you managed to buy Batman: Arkham Knight before it was removed from sale last week, and if you don't mind grappling with the still-broken game, then a new mod unlocks ten playable characters in Arkham's freeroam mode. Red Hood, Harley Quinn, Nightwing, Robin, Azrael, Bruce Wayne, Catwoman, Commissioner Gordon, Joker and GCPD Officer Owens are all playable if you install the mod shown in the video above.
Installation instructions are in the YouTube description here. I haven't tested it myself (make sure you run a virus scan and backup your files), but the footage is interesting on its own terms. If you'd prefer to wait for the game to be completed, Rocksteady has announced it is "working like crazy" to mend the PC port, and the first of many patches went live at the weekend.