It has already been previously rumored that Brainiac will be the main villain in Zack Snyder's first Justice League film. Well, today Badass Digest has corroborated that story to reveal that Brainiac will indeed be summoned to earth after the beacon in Man of Steel is activated by Clark. They go onto reveal the roaster that will appear in the first Justice League movie which will include Superman (Henry Cavill), Batman (Ben Affleck), Wonder Woman (Gal Gadot), Cyborg (Ray Fisher), Aquaman (Jason Momoa) and The Flash (Ezra Miller). All of whom will make an appearance in Batman v Superman: Dawn of Justice.
Now you maybe wondering where Green Lantern (Yet to-be-cast) will fit into all of this. According to the site he will be off planet during the events of Justice League 1 but at the end will come back to earth to warn the League of an even bigger threat Darkseid!
The site also goes onto mention Justice League Part's 1 & 2 may be shot back-to-back but that has also been reported for Batman v Superman and the first Justice League.
Justice League Part 1 hits Summer 2017