Back in June, James Gunn confirmed that David Corenswet and Rachel Brosnahan had been cast as Superman and Lois in his first "new DCU" movie: Superman: Legacy. We've seen several relatively minor roles been cast since then, but one of the most important ones was still missing...until now.
Usually very reliable Justin Kroll and Anthony D'Alessandro over at Deadline claim that Nicholas Hoult (X-Men: Future Class, Mad Max: Fury Road, The Menu, Renfield and more) has been cast as Lex Luthor in Superman: Legacy. It's worth mentioning that The Hollywood Reporter's Aaron Couch reports that Hoult is still just in talks to play the role.
It would make some sense, however, as Hoult was also one of the finalists to play Superman, so it sounds like Gunn and crew really like him.