Superman VS Batman

Just for fun and because I had ten minutes of free time to devote to this geekiness bit of MS paint project.

I completely understand that overloading a movie with too many characters (specifically villains) can ruin a movie.  But that's because it became formulaic in the 80s and 90s.  Everyone tried to put their own spin on Burton's Batman pattern of a super hero tale/franchise. 

It can be done well though, as MARVEL continues to prove.  DC has a lot to prove with their attempt.  Batman VS Superman (which should be changed to World's Finest)  has already released a massive list of heroes that will appear in one form of another. 

I think a villain team up could be amazing.  They're going for it in the Spidermovieverse.  Plus I think you'd truly need this many villains to prove a serious threat to this team. 

Who would you cast?