I'd like to see Brainiac in the sequel, and have him discover and use the Eradicator device to try and corrupt Superman and use him to take over earth. This would somehow result in the humanoid Eradicator showing up and battling Superman, with Supes seemingly destroying it and then taking down Brainy temporirily, but then getting infected with Kryptonite or something so that Supes eventually dies and the movie ends in a cliffhanger. But then at the beginning of part 3, the Eradicator awakens (because his purpose is to preseve Kryptonian life) and he revives Superman, but Supes is in a coma, so Eradicator subs in temporarily as Superman. Then when the real Supes is back to his old healthy self, they team up and take down Brainiac and Lex (who have both returned to lauch a new scheme, possibly one involving Doomsday). That would be my ideal Superman trilogy.