The sequel to Guillermo del Toro's 2013 hit, March 23's Pacific Rim Uprising takes place after the war between Kaiju and Jaegers. In Pacific Rim 2, the aliens known as Precursors play an integral part in the war of the worlds again, and even though Kaijus from the Anteverse beneath the Pacific Ocean seemed like the biggest threat in the first movie, the Precursors actually had a lot more to do with the giants' war than you might have originally understood. In the sequel, which stars John Boyega, Charlie Day, and newcomer Cailee Spaeny, viewers learn that there may be more to the aliens on the other side of the portal still hold a lot of power.
Spoilers ahead! As you probably remember, Kaiju are the ginormous amphibious monsters that battled against the human-operated robotic Jaegers in Pacific Rim. But they weren't operating on their own accord. Instead Precursors, which are sea-creature looking aliens from the underworld, both created the Kaiju and opened the portal beneath the Pacific Rim that freed the monsters in the first place. Precursors want to take over the world, and even if the Kaiju do their battling for them, the Precursors control them.
In Pacific Rim Uprising, you learn that Kaiju can share their thoughts with each other. That means that both the Precursors and humans might be able to control all of the underworld's monsters at once.
In an interview for the blog Bleeding Cool, del Toro explained that the Kaiju's hive mind would become an important factor for Pacific Rim Uprising. Said the producer,
There was a line that I deleted from the [first] movie that will come back if I do a second movie. Newt explained that the Kaiju are not carbon based organisms like humans, they are silicon based. The only part I left of his explanation is that they have a hive mentality, meaning that if you “drift” with a Kaiju brain, you are drifting with every Kaiju alive.
"Drifting" refers to the neural connection that humans make in the Pacific Rim films that allows them to combine their thoughts with one another. While captaining a Jaegar, the pilots must meld their thoughts to act as one. Even though del Toro didn't end up directing Pacific Rim Uprising, his explanation to Bleeding Cool still plays a key part in the film. If humans can control Kaiju's minds, that also means that Precursors might be able to control a human's mind should someone drift with Kaiju.
In the sequel, it becomes increasingly clear that Precursors are the greatest threats to humanity. Sure, the ginormous sea creatures that can take down a skyscraper with the flick of their tails pose a great threat, but the Precursors from the Anteverse will likely not stop invading until they are stopped directly.
At the end of Pacific Rim Uprising, the military attempting to defeat the Kaiju resolves to adopt a more proactive, offensive approach to fighting the Precursors. Whether or not a battle will happen in between Precursors and humans will take place in a third Pacific Rim movie is yet to be determined.
In an interview with Collider, del Toro said that he and the filmmakers "decided that we’re going to shoot ambitiously and say, ‘Let’s hope we have three movies.'" Should a third Pacific Rim movie come out, the aliens who created Kaiju will definitely offer another threat to humanity, but the humans might have to take on a more offensive approach. As Boyega told IGN, "We'll probably have to go to their world [next] time."
It sounds like a battle in the Anteverse between humans and Precursors could definitely go down.