Apart from Batman and Superman, BvS will also have Aquaman, Wonder Woman, Cyborg and Flash. Zack Snyder tried to introduce all of them in the same movie at once. But I am unable to make a sense of how they will be introduced for the first time in this new cinematic universe? Will they have a little origin story in this movie? They are not even shown in recently released teaser:
So, how Snyder will fit them in a story that is targeted totally on fight between Batman and Superman?
As Snyder said before that this movie is based on The Dark Knight Returns. But in that movie there was no sign of these superheroes. So, after analyzing all these questions and trying to find their answers, I tried to make some sense and came up with some theories:
1. Wonder Woman
Wonder woman will come to Earth and will warn about some threat from outer space. This threat will be introduced in Justice League movie. (I hope it will be Braniac because he is so awesome :D). The government won't believe her and will see it as a trick to dominate humans. Even Batman won't believe them and Superman will join Wonder Woman's team and will try to convince Batman and the government.
Government will decide to send these aliens back to their home planet and they will use Cyborg with Batman against Superman and Wonder Woman. A summary of Cyborg's origin will be included in this movie.
Aquaman will have a little origin story at the end. He will be training with his father and will become Aquaman at the end of the movie to help form Justice League and fight the upcoming threat.
I am unable to fit Flash in this but Batman might call Flash to assist him fight with the Superman. And I don't think the origin story of Flash will be included as it is already done in the series. And will be rebooted in upcoming Flash 2018 movie.
So, this was my personal view of how the story can make sense. If you have your own theory then please leave it in comments. Thanks for reading. :)