My two guesses would be either Darkseid or Brainiac.

My guess is Mongul.

I would guess Darkseid, Brainiac or Mongul

Darkseid because he's one of the most notable "epic" villains in DC

Brainiac because there were a few hints toward him during the first Injustice in the backgrounds

Mongul because the trailer makes it seem as though this is some sort of gladiatorial thing going on. It would make sense and fall in line WarWorld.

It would definitely have to be some sort of extraterrestrial threat - the time that the heroes spent fighting among themselves would undoubtedly leave the Earth open to attack from hostile forces outside.

Trigon

Please let it be Braniac. Don't get me wrong, I like the imposing nature of Darkseid, but it's so on the nose. He's the vanilla villain because everyone expects him. Braniac is a bit more of a schemer than Darkseid, looking to minimize opposition before the battle even starts. I like that and I think it would make for a good story mode.

The Big Bad for Injustice 2 will be none other than those damn Star Labs Missions, if they come back that is. :)

There are people saying the voice of the first trailer is Ra's al ghul's, and while I don't see him being the main villain, I could see him suporting Superman and trying to release him, working then with him until someone bigger and badder like Darkseid or Brianic shows up.