I like your thinking there. What about a tie-in with Guillermo del Toro's JUSTICE LEAGUE DARK? There hasn't been much discussed about that movie.
who would be the villain
Whoever it is, it looks like Del Toro is actually looking for a cohesive universe.
Del Toro revealed in an interview with MTV in July 2013 that he hopes the DC Cinematic Universe, which started with Man of Steel, will become as cohesive as the Marvel Cinematic Universe, and he added that if there was any correlation to that universe and this film, he would honor it.
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I do think they like his idea for that movie but it would happen after the first JLA movie