The Justice League film does have lots of trailers that reveal a lot about the film, but there are also a lot of things about the film that aren't fully revealed. I have predictions for 5 characters in the Justice League film.
I heard about Superman wearing a black costume in the film so I think that he's being brainwashed by either Steppenwolf or Darkseid into being evil which could bring back Batman's obsessive hatred of Superman.
Batman V Superman's version of Batman was supposed to be a meaner version of Batman on purpose due to the recent tragedies he dealt with. I think Batman will be more friendly and optimistic in the Justice League film despite acting like a wise guy in the film's trailers.
Steppenwolf is apparently the film's main villain who wants to destroy the world. However I think his real desire is to either retire or even get destroyed by the Justice League. In a interview with the actor that plays Steppenwolf mentioned that he's exhausted due to his age and how demanding Darkseid is. I have 2 predications for what his main goal is: I think that he will either try really hard to destroy the Justice League so he can finish doing his job and be rewarded with a relaxing retirement or he will try to get 1 or more of the Justice League members to destroy so he won't have to deal with Darkseid anymore.
I think that Darkseid will show up in the film. It would be weird if he didn't show up considering that he is 1 of the Justice League's main enemies and is Steppenwolf's nephew. I doubt that he will be in that many scenes. He could secretly be the film's real main villain or he could have cameos. Either way I hope he gets a memorable role.
Lex Luthor Prediction
I've heard that apparently Lex Luthor got cut out of the film I was upset about, but considering that he has somewhat of a friendship with Stepphenwolf I think that he will show up. Like Darkseid his role will likely be short, but I hope he gets to be a scene stealer again.