I want to see Vandal Savage as the main villain!! The Justice League vs. The Illuminati; that would capture my imaginations like no other. They could call it "The Light" as they do in Young Justice, but I personally think "Illuminati" is way more bad ass and scary.
I feel like that's as realistic as you can get with a JL movie. In a world where there's a single cabal of individuals who have been controlling the events of human history (Savage being the leader), it makes sense for a band of superheroes to join forces to stop him. There's something about the Justice League that just lends itself to conspiracy theories - I hope conspiracies become a big part of this film series.
I say save the alien invasion for the final film.
I feel like that's as realistic as you can get with a JL movie. In a world where there's a single cabal of individuals who have been controlling the events of human history (Savage being the leader), it makes sense for a band of superheroes to join forces to stop him. There's something about the Justice League that just lends itself to conspiracy theories - I hope conspiracies become a big part of this film series.
I say save the alien invasion for the final film.

t:
)...Of coarse Batman being the only one to show up in all three, haha.