I thought about this and who would be a presence on the screen? Lex Luthor demands someone who can carry. I came to the conclusion of Bradley Cooper. I know Marvel has their hooks in him, but he qualifies since he is a living actor. If you think about it... Limitless, on top of the world and then going crazy. Silver Lining Playbook, trying to be good but fails to understand why he is not embraced by everyone. There were a lot of human qualities you could latch on to with someone who clearly was mentally unstable.
Bonus: I seem to be stuck on Deathstroke. He is my all time favorite villain. But who would play him? Only one man could make him scary and methodical as he should be... Javier Bardem. We have seen it in No Country for Old Men and Skyfall. He would be the only person I would trust not to ruin my favorite DC Character.