Metallo is who I WANT! He can be the Two-Face of the MOS movie. A henchmen for Luthor who turns into an antihero near the ending, but with a tragic backstory related to some accident (as John Corben).
Metallo can add intensity, gravitas, and DEPTH to the movie.
Metallo should also SEEM like the main villain, with Lex cropping up a few places in the movie, and being revealed in full during the last scene with Superman/Metallo and Lex in a 3 way stand-off of sorts