Re: Who should be the villain in the sequel for Man of Steel?
I don't think Lex should be even a secondary villain in this film, much like Joker he should be saved for the solo franchise and not in a team up film. Having said that, I do believe Lex should be in the film and have an important character arc, he should be introduced as a philanthropist and Superman-sceptic genuinely trying to rebuild Metropolis (along with fellow philanthropist, Bruce Wayne), similar to the Lex Luthor we see towards the end of Superman: Earth One II, and throughout the course of the film the events turn him into the Lex Luthor we see in Luthor: Man of Steel for MOS 2 and later All-Star Superman for MOS 3.
My preference for villains could be a version of Metallo (perhaps built for the military by Lex) and maybe Riddler (maybe he is unearthing the military conspiracy). Characters that haven't got modern incarnations, but not the arch-enemies of the characters.