Darkseid is set to be the main villain.
"After some persistent snooping, we have exclusively learned that the main baddie is none other than…
Yes folks, Darkseid. The supreme monarch of the planet Apokolips and the greatest threat to the DC Universe.
It’s kind of obvious if you really think about it. I mean for
Superman, Batman, Wonder Woman, Green Lantern, and others to come
together in a summer tentpole movie to face a threat, well, that threat
better be planetary. Who were you expecting? Lex Luthor?"
With no Warner Bros confirmation yet, we should just treat this as the rumour that it is. Darkseid is a pretty bad ass character for people who don't know about him, plus with the recent reboot DC went through Snyder and his co will be loving this an having the opportunity to mix it up like he did with Superman i.e no over the suit boxers. Darkseid was the first ever villain that the Justice League faced together in the comics, so I can't see there being a more fitting villain or them to square up against.