Not to interrupt the discussion which has nothing to do with the thread title, but I just had a thought about the villains of Superman/Batman. So everyone agrees there needs to be a secondary villain other than Lex who can provide a physical challenge for Superman but the major ones who do such as Braniac or Metallo would probably be best saved for a solo Superman movie. So what about the Elite?
Superman could decide on his policy of never killing again after the events of Man of Steel, perhaps publicly announcing it in an interview with Lois, which could lead the public to lose faith in Superman's ability to get the job done. Suddenly this new group appears who will get the job done. Superman can be busy fighting them and he asks Batman to investigate who they are and who is funding them. They are powerful enough to provide a physical challenge to both Superman and Batman at the same time and have interesting powers so the action scenes would be really cool but they also wouldn't overshadow the main villain, Lex, and they also don't require that much setting up, while also providing an interesting moral dilemma for Superman. Lex meanwhile could be running a presidential campaign based around preventing the destruction caused between superhumans, such as in Zod's invasion and the conflict between Superman and the Elite. In the end Batman could find out that Lex was behind the group and he and Superman could confront him, saying they can't prove it but they will be watching him. Also, Manchester Black is a bad-ass.