Here is a quick poster I made of Joaquin Phoenix as Lex Luthor.
I can't see Phoenix signing onto a film like Man of Steel/ Justice League. But you never know, as I didn't think that Ben Affleck would take the roll of Batman! (But I am so glad that he did!)
Idris Elba would be my 2nd choice for Luthor (but due to him recently being in Thor, I think it would be very unlikely that he would come over to the DC universe).
Let me know what you think!