While we're waiting on the Season Pass to hopefully give us control over Arkham Knight's Bat-friendly roster for longer than a quick dual-play or short cut-scene, a PC modder has found a way to let you access and free roam Gotham as 10 other characters from the game.
Note, as some of the characters listed are potentially spoiler-worthy if you've yet to play the game (or are only a few hours in), we won't list them here. We're merely direct you to the demo video as the modder puts the cast through their paces. Again: spoilers in the link.