I like Mark Strong as an actor, but I'm against the idea of him as Lex. It just seems like uninspired casting. Strong always makes a great villain, but personally I'd like to see him play against type at this point.
You could argue that Strong was the best thing about
Green Lantern and that we never really saw him as a villain in that film, so doesn't he deserve the chance to play an equivalent arch-nemesis role in a better film? But I don't want Lex to be such an obvious villain. I know Strong could give him subtlety, but still, as soon as a I see Mark Strong in a role my first assumption is "villain."
My preference is for Luthor to have a bit more of an attractive allure. I think it would be intriguing to have Lex portray himself as more of a heroic type to the people of Metropolis, helping repair the damage that this "alien" caused and earning their goodwill. Superman would know what Lex was really like, but this would make it that much harder for him to convince the public that he, not Lex, was the real hero.
In that regard, I would want Lex Luthor in MOS 2 to be handsome and charismatic -
and, this should go without saying, confidently rocking the bald look.
Now, when it comes to who would portray this magnetic figure? Damned if I know. I did think Denzel Washington was at least a more interesting choice than Strong. Daniel Craig would probably be good, but how's his American accent?
I still feel like there's some out-of-left-field casting choice out there that will initially surprise us and then come to seem brilliant in retrospect (and yes, I am aware this would perfectly mimic what happened with Heath Ledger as the Joker

).