I would recast a whole lot, to be honest. Actually, at this point in time, I somewhat wish we could jettison the Snyderverse into oblivion, and start over. It's obviously not going to happen though.
On the subject of Lex, it's always difficult to please everyone, simply because he is a character who's had many onscreen renditions in the past: you had crazy scientist (which is what I think Eisenberg was going for), eccentric real state billionaire (Hackman/Stacey), the thug-in-a-suit (e.g. STAS), and the more composed and calculating businessman/politician (e.g. Young Justice). I would prefer to see the latter, not only because I think he is a more formidable foe that way, but also because I think Lex Luthor could make some legitimately intringuing points: what makes Superman's morals the "right" ones? Should he go around imposing that on everyone, just because he can?
So I would cast someone charming and elegant for the role, a more seasoned figure that could conceivably make all his questionable dealings and yet escape suspicion and prosecution, keeping the public's trust in him. Perhaps they could even follow up on some variation of the President/UN Secretary-General Luthor storyline. One suggestion I really like is Iain Glen:
Glen can effortlessly pull-off the intellectual, condescending tone while being sufficiently charming and composed. He is British, but I'm certain that he would be able to emulate an American entonation for Luthor.