Superman needs to die. I mean literally. There needs to be a movie where Superman dies. The Death of Superman comics were awesome (from what I remember - it's been a while). soda's going to need to help me out with this, but I believe he died in an epic fight with Darkside. I think the circumstances following his death would have to be changed to be more cinematically fitting, but it's a good start.
I don't have a problem with Lex Luthor being in future movies, but I agree with River Dog, he's been done to death. He shouldn't be the main villain, but more of a side character like Scarecrow/Rahs Al-Ghul was in Batman Begins. He's Superman's archnemesis so I don't have a problem with him being in Superman movies, but the movie cannot be about him trying to rule the world through some kind of natural disaster or bomb or something.