I'd love to expand on the Luthor family. His niece Nasty was a scene stealer in All Star Superman, and I bet fun stuff could be done with her. She can play at being a super villainess, date a teen super hero, work with/against Lex. Officially, Nasty was the daughter of Lex's older sister we never saw on screen so there's another character that could be introduced. Anyone with Lex for a brother who names their daughter Nasthalitha sounds like an interesting character.
I'd go ahead and bring back Lex's daughter Lena too. There's a lot of potential for her, she's close in age to Jon so they can do a friendship or some interesting dynamic with them. Lex as a father adds complexity to his character. Nasty_Luthor_001.jpg