Batman v Superman: Dawn of Justice is so much more than just the Caped Crusader and Man of Steel meeting for the first time in live action. The movie is also getting the ball rolling further on expanding the DC Extended Universe. As the "Dawn of Justice" part of the title suggests, this includes foreshadowing the formation of the Justice League, and director Zack Snyder implied that the 2016 blockbuster almost serves as the first entry of an unofficial trilogy.
With Justice League: Part One preproduction having already started and filming set to begin in a few months, Snyder told Obsev that the movie will be the next step forward for many of these heroes. As he put it:
Well, listen, we're working on Justice League right now, right? In a weird way, Justice League is the next step for these characters to form a bigger team. Maybe there's a bigger enemy to fight?
Batman v Superman: Dawn of Justice is already throwing a powerful baddie onto the board to get the DC Trinity together. Batman, Superman and Wonder Woman will be working together to defeat Doomsday, but when Justice League: Part One comes out, they’ll be teaming up with Aquaman, The Flash, Cyborg and reportedly Green Lantern. Aside from Green Lantern, all of these characters will also be appearing in Batman v Superman: Dawn of Justice, but their roles are so minor that it won’t be until Justice League: Part One that we really get to see them in action. Of course, the last entry in this "trilogy" would be 2019’s Justice League: Part Two, which will see the main team return and maybe even some extra additions. If you’ve read the comics or watched the cartoons, you know the team is more than just the original seven founders.
As for this "bigger enemy," there are only a few select individuals from the comics that have been powerful enough to bring the Justice League together. Brainiac and Darkseid have previously been rumored as contenders, but ideally 2016 will be the year we learn who this antagonist will be. Typically these heroes operate on their own, so it’s only the direst of situations that would get them to work as a team. You need a truly world-threatening villain to get the superhero team formed. As this trilogy progresses (and probably even beyond that, taking into account what DC might have planned post-2020), moviegoers will see this team deal with other antagonists threatening Earth, and with each mission, they’ll become a more effective fighting unit.
Batman v Superman: Dawn of Justice will lay the groundwork for the Justice League on March 25, and the team itself will officially be seen when Justice League: Part One arrives on November 17, 2017.