There's a reason Batman v Superman: Dawn of Justice went with that slightly wordy title. In addition to featuring a showdown between the two DC heavyweights, Zack Snyder's film was largely created to help set up a bigger DC Extended Universe. How exactly did it do that? We've got you covered.
Major spoilers follow.
In Batman v Superman, it's revealed that Bruce Wayne has been Batman for 20 years. In that time, he's remained largely a mystery to the public, but two decades of crime-fighting leaves a lot of history, and we get a few glimpses of it in the movie. First, there's the suit of his old sidekick Robin in the Batcave, with the words "Hahaha Joke's On You Batman" written on it. Second, Wayne mentions Gotham's history with people dressed as clowns, a clear reference to the Joker. Third, Wayne Manor has been burned and is uninhabitable, forcing Bruce and Alfred to live out of a nearby lake house. The implications here, at least of the first two, are that the Joker killed Robin in some kind of public struggle, but that's just an educated guess. Whether or not we'll get explanations of any of these things in future movies, we don't know, but there will certainly be ramifications.
The Meta-Human Theory
This is the big one, so we're going to break it into a few parts. The reason Lex Luthor wants to get kryptonite to stop Superman is because he has a theory that other people like Superman exist and humanity needs to be protected from them. This is a theory that Senator Finch is also aware of. Later though we realise this isn't a theory at all and, Lex has video proof these people exist. Luthor also hints that his "R&D is up to all kind of no good." Is he referring to meta-humans?
In the middle of the film, Bruce Wayne has a dream where he's trying to get kryptonite in a post-apocalyptic world. However, he realises he's been set up by an army of Superman worshippers. There's a huge Omega symbol burned in the Earth and Bruce is attacked by parademons. Dreams, generally, come from the subconscious and while his fear of Superman makes sense, the meaning behind that Omega symbol (more on that in a bit) and parademons are a bit harder to explain. Is this a premonition of what's to come? Undoubtedly.
A Flash Between Dream and Reality
Immediately after the Knightmare, Bruce Wayne sees something. A white light with a red being in the middle of it. This being, almost certainly, is Ezra Miller as the Flash travelling through time. Travelling form where and for what reason? We don't know. We also don't know if Bruce took any of what he said to heart, but here's what was said.
Flash says that "Lois Lane is the key," that he "came too soon," "You were right about him," and "You've got to find us." Lois Lane being the key could refer to this movie (where Lois finds the kryptonite spear) or something in the future. That he "Came too soon" is probably a reference to the fact Bruce doesn't know about The Flash yet and isn't yet looking to recruit the Justice League. "You were right about him" may mean that Superman does something bad in the future, or it could be that Lex Luthor is a bad dude, or something else. Ay any rate, "You've got to find us" is the one bit of advice Bruce does seem to take by the end of the movie.
Lex Learns Secrets From a Kryptonian Ship
When Lex boards the ship abandoned after Man of Steel, the computer tells him it has information from 100,000 galaxies. To which Lex says, "teach me." We don't know what, exactly, he learned, but we think that's how he found out how to make Zod into the killer Doomsday, and how he may have made contact with something beyond. Something worse. Something that uses an Omega symbol (I promise, we'll get to that). This scenario will also serve as a nice tool for the future screenwriters to explain how Lex knows anything.
Wonder Woman's History
When Bruce Wayne accesses Lex Luthor's encrypted files, he discovers a bunch of things. First is the photo that Diana Prince, a.k.a. Wonder Woman, says Lex Luthor possesses but which doesn't belong to him. That photo shows her in battle during World War I, alongside several other characters. The inclusion of Chris Pine almost certainly means we'll see this photo taken in the Wonder Woman solo movie.
The Justice League
So we've got Batman, Superman and Wonder Woman. What about the rest of the Justice League? Well, when Diana Prince opens the other files from Lex's computer we see a man who can run really fast called "Red Streak," a man living underwater in the Tonga Trench, and D.r Silas Stone creating a hybrid man and machine at S.T.A.R. Labs. That's the Flash, Aquaman and Cyborg. Plus, Stone has what appears to be a Mother Box, a piece of technology that ties in with a major character teased in the film.
In Batman v Superman, Superman dies. Twice. Well, at least once, but maybe twice. The first time is when a nuclear explosion hit him in outer space; the second is when Doomsday stabs him in the chest. After the first one, the rays of the sun bring him back to consciousness and life. As the second, we don't know quite yet. We just know that Clark Kent is buried in Smallville, an empty coffin is buried in Washington D.C., and a memorial to Superman is placed in Metropolis. The final shot of the movie is dirt rising on Clark's coffin, suggesting something is happening to his body inside. Also suggesting this resurrection? Every time Henry Cavill talks about being in Justice League.
Bruce Wayne and Diana Prince Will Scout the Justice League
At Clark Kent's funeral, Bruce tells Diana that it's time to find the other people like her because they need to fight. Why will they need to fight, asks Diana? "I have a feeling," says Bruce. Where could that feeling come from? Well, here's one last connection.
In the film's final moments, Bruce Wayne threatens a now bald Lex Luthor in prison and Lex tells him that the bell has been rung. Someone knows that "God is dead." "He's hungry and he's coming," Lex warns. The film then cuts to Luthor's dad's painting, which shows a battle between heaven and hell. The painting has been flipped so that Hell is coming from the sky. Earlier in the film, this was a reference to Superman; now it's a reference to something greater -- something that uses an Omega Symbol as its indicator. Something named Darkseid.
Darkseid is basically the DC Extended Universe's equivalent of Marvel's big bad Thanos (although Darkseid was created by DC Comics first actually making Thanos Marvel's Darkseid). Darkseid is an unstoppable, god-like character from the planet Apokolips who wants to eliminate free will from the universe. He's a formidable villain who may end up showing up in Justice League movie, scheduled for release in November 2017.