i believe they have said they are going to make three of them and will most likely have lex luthor as a villain. a way i would enjoy him being the villain is the man in the shadows pulling the strings. have him creating metallo and using him to fight superman. bring in some new villains like doomsday, bizarro or brainiac.
i have a little feeling that the man of steel will have a cliff hanger ending. a little easter egg to who the villain might be in the next movie or Zod being defeated and leaving possibly coming back in the third film