Sam Rockwell
Sam Rockwell never turns in a bad performance, and even when he is an uneven film, his presence makes it wholly watchable. From what I have heard of the casting so far, this next Batman movie is going to need an actor like that to offset the Affleck-Eisenberg douche factor. Sam Rockwell would be perfect. An ounce of credibility in an otherwise incredulous world.
The other thing that is amazing about Rockwell is, you can’t quite be sure what direction he would go with his Joker. Would it be more Heath, or more Jack? Would it be more “early prince of crime” Joker, where he is kinda funny and erratic? Or would it be later versions of the Joker from the comics, where he gets his skin ripped off and is just an insane motherfucker? Either way, if Rockwell was playing it, it would work. I guarantee it.