I don't need Clark and Lex to have been friends (or even friendly acquaintances) in Smallville. The less Lex knew of Clark growing up, the less likely he would ever figure out The Secret (which absolutely, positively should never happen "for realsies" as it did in DAWN OF JUSTICE).
I do need adult Lex to treat Superman and Clark Kent completely differently. Lex should treat Superman with consistent open contempt and an irrational hatred born of pride and fueled by jealousy. That never changes.
But Lex should treat Clark with a weird sense of magnanimity. Lex should erroneously view Clark as an ideal example of all the Humans who would surely follow Lex's lead if only those treacherous super-heroes didn't exist. And Clark would tolerate it, if only to keep a better eye on what Lex was doing.
I was thinking, Lex should be one of those child prodigies who went to college at a very young age,
like this kid is doing in real-life.
I could see a married Lionel Luthor siring Lex upon one of his many mistresses. Lionel dismisses the mistress, never believing she might get pregnant. Mistress gives birth in secret, dies shortly after due to complications of childbirth, Lex is then raised by abusive foster parents whom he later kills via car "accident"; as a 9-year-old genius he graduates from high school and gets a full ride to Harvard, where he gets his B.S., M.B.A., Ph.D., J.D. and M.D. over the next 10 years.
Seriously, out of all the real-life American universities out there, Lex Luthor would definitely have attended Harvard University, right?