One good thing about details regarding Dawn of Justice and the Justice League films being so scarce is it leaves plenty of room for fans to speculate. Personally, I'm very excited to see who the major villain to face the Justice League will be. Whoever it ends up being will likely be instrumental in bringing the team together in the first place.
My top pick for the first major threat to the League is everyone's favorite 12th level intellect, Brainiac. He easily poses a great enough threat to force Earth's mightiest heroes under one banner. I think Ralph Fiennes would be a top casting choice for the robot bent on ultimate universal knowledge. He's got a great, naturally unflappable demeanor, and even proved a touch frightening in Red Dragon (I also think he'd make a great Victor Fries or Alfred Pennyworth, but that's another discussion entirely).
Brainiac and Superman also have something of a history, which could be interesting for Supes to uncover as Brainiac reveals he knows more about Krypton than Superman does, and Superman discovers - and possibly visits -Kandor. And whether Brainy is attempting to shrink cities, the entire planet, or just assimilate all of Earth's unique knowledge before destroying it completely, all of Leaguers would have a stake in the fight.
Who should play Braniac?
Something else that could be interesting is bringing the Greek gods into the fray. If Ares or Hades (or any of them, I just think they're the two most likely to cause trouble) gather too much power, Wonder Woman could conceivably be led to call on the rest of the League to help take them down. I won't fancast the entire pantheon here, but I think Dennis Haysbert would be a great candidate for Ares, with his spectacularly deep voice and large, imposing frame. Though, usually not seen in intimidating, villainous roles, I think he played Manute well enough in Sin City: A Dame to Kill For, and could pull off the God of War, bent on plunging the world into eternal conflict. As for Hades, if he were based closer to the New 52 incarnation, I can think of no one better than Jack Gleeson, who played the boy king Joffrey Baratheon to a great deal of acclaim in the Game of Thrones TV series. He'd be a perfect fit for the insecure, hate-filled, childlike God of the Underworld, who could be attempting to expand his dominion by wiping out all life on earth.
Dennis Haysbert as Ares
Jack Gleeson as Hades
What do you lovely people think? I'm just spitballing here, I'm sure there are better ideas for who the cinematic Justice League could face off against, and if you've got any, I'd love to hear about them in the comments. Thanks for checking this out.