1) Red Hood
With the Batman V Superman: Dawn of Justice trailer confirming that The Joker killed Robin in the DC Extended Universe, a logical place for the Caped Crusader’s solo movie to go would be an adaptation of the Red Hood storyline. That saw Jason Todd (who was resurrected in the comics in the most convoluted fashion possible) return to Gotham City in the persona originally held by the man who killed him. Now a vigilante similar to someone like The Punisher, the Red Hood took aim both at The Joker and the mentor he felt had let him down by not killing the villain in retaliation.
[zergpaid]Unfortunately, DC Comics turned Jason into a complete lunatic before The New 52 reboot transformed the one time Robin into an anti-hero of sorts. Following the latter path would be a smart move for Warner Bros. as they could then consider launching a spinoff series of movies starring the character.
Going back to his initial return though, and the Red Hood would be a great villain for Affleck’s Batman to square off against. Sure, he may not be as iconic as some of the others listed here, but it would result in the most personal Batman movie to date, and that’s worth getting excited over.