This is no brainer and would be the casting coup of the all casting coups in the history of comic book movie films. Washington is the definition of an A-list actor and would bring a whole different level of attention and fan base to the MAN OF STEEL sequel. I would consider Washington getting/accepting the role to be in the same area as Gene Hackman being cast in the original SUPERMAN film in the 1970s. Like Hackman, Washington is a well respected award winning actor who tends to elevate the films he appears in merely by his presence alone. The one thing Washington has over Hackman is that Washington is more of a legitimate leading man and box office bankable star so he would be a huge get.
As for how he would be in the role, I think the best role to look at for a comparison would be Washington’s Academy Award winning turn as Det. Alonzo Harris in TRAINING DAY. In that role Washington seamlessly transferred all of his innate charisma and magnetism from the upstanding good men he was known to play into a manipulative and malevolent bad guy who was capable of almost anything. Bringing some of that spirit to the role of Lex would be great and make for a pitch perfect Luthor in my opinion.