Lex has to get his deserved share of character development. Being an evil dude for no reason, or as if he was born with the evil gene is boring and unimaginative.
I'd like to see Brainiac in the first film. If he is given a Kryptonian origin like in STAS, it could help us examine Krypton's history even more and explain why nobody else survived (Brainiac sabotaged any escape attempts). He would also provide amazing fight scenes.
That would give Lex time to evolve during the first film and become the evil mastermind in the second. Lex's problem with Superman is that he doesnt trust him and he believes that deep down Superman wants to rule the world. But even if Superman is benevolent, Lex believes that he is spoon feeding the world, being too helpful and not letting it evolve on its own. But we all know that deep down Lex is jealous that Superman is blessed while no matter how hard he works he will never get the same attention and acceptance from the public.
That is why i like how STAS did Lex. He was good at first but when Superman stole the spotlight, he went nuts. That and all that "i dont trust him" stuff.
And with another alien (brainiac) threatening to destroy the world, Lex's fears over Superman's motives would be more than justified. And if the world doesnt understand the threat that its facing, then its up to Luthor to protect humanity. So he changes the direction of his company, from charities and whatever else its doing, to arms manufacturing, lasers, kryptonite, *****ebaggery and research for the cure of baldness.
And one more thing: I hate the version of Lex that grew up in Smallville being friends with Clark. Seriously... goddamn!