A Free Roam mod built into the Arkham Episodes menu. The following characters are supported:
The Joker (First Person)
Red Hood (Only works if the Red Hood DLC is bought)
Harley Quinn (Only works if the Harley Quinn DLC is bought)
Batgirl (Only works if the Matter of Family DLC is bought)
To use the mod, put the FREEROAMMENU folder in \steamapps\common\Batman Arkham Knight\DLC\313100, if you have it. Go to the Arkham Episodes menu. Scroll down and you'll see the new "story packs". Start one of them (they shouldn't interfere with the main story save file). Restart from checkpoint for thugs to appear.