It has been rumored, if not confirmed, that Brainiac will be the main villain for Part 1. However, because of AOU's Ultron have similarities with Brainiac, it has been suggested, if not speculated, that perhaps Brainiac should be replaced. Now, for me, I would want Brainiac in Superman, not JL, it's only fair, but I felt I knew why as given to one of my ideas below. Who or what could be a replacement?
I've been doing a Marvel Marathon in preparation for AOU, and one thing I had was based on Loki; not actually Loki, or Loki-like, but the idea of having a villain from someone's rogue's gallery. And this is most likely the same idea for Brainiac, from Superman's rogue's gallery, and that would make sense; but, I feel it should be a more human villain.
Ra's Al Ghul is one idea, especially going from BvS into JL, but we need someone different, and it's not just Superman or Batman, there's also Wonder Woman. What about Ares, the God of War?
The other idea would be Lex Luthor's Legion, making up the enemies of the heroes in that movie, thus banding them together creating the JL. Obviously, people would think of Superfriends (maybe), but the heroes who have appeared so far are Batman, Superman, Wonder Woman, and Aquaman: the Superfriends.
Now yes, the perfect villain is Darkseid, and that's true. Use all of his versions, have them be related to each team member, and it unites them. But should he be saved for movie #2 as they intend? Or if not #2, #3 at least?
Thoughts?