If you (still) have a copy of the PC edition of Batman: Arkham Knight and you have an adventurous spirit, there's already a mod out that lets you play as 10 diffferent characters in the game's free roam mode.
Of course, one must realize that the official, unmodified version of the game has issues so severe that developing studio Rocksteady warns that significant work still needs to be done to stabilize it, following the release of a patch for PC yesterday.
Nevertheless, this mod by Wastelander121 and MrJAG allows players to cycle among Red Hood, Harley Quinn, Nightwing, Robin, Azrael, Bruce Wayne, Catwoman, Commissioner Gordon, Joker, GCPD Officer Owens (and, of course, Batman) during free roam. The mod is available here, and the instructions also may be read in the YouTube video's About section.
Plainly, one should back up the game and their save files before introducing this unofficial modification.
Batman: Arkham Knight is very much like Two-Face, in that the console edition of the game has gotten very high scores (we gave the PlayStation 4 version a 10) but the PC port, handled by Iron Galaxy Studios, has been so broken it was delisted from Steam.