Author's Note: this prediction is based off a few tweets, some set leaks, and two trailers. It is far, far too specific to be at all accurate, so I suppose "Florida Man fabricates a complete plot out of what little is known currently about Ninjago: United" would be a more accurate title, but it's less fun. So, basically, don't read this and expect actual predictions I think are 100% correct, since I'm really just building my own story out of the few pieces we've been given.
So it's been a year or so since the events of Crystallized, and the ninja are doing fairly well for themselves. There's still crime, still trouble, but it's nothing they can't handle. Then, something in the energy of the universe begins to shift. Master Wu calls Lloyd to help him. He says he worries about something called "coalescence," which is basically the Ninjago equivalent of the Big Crunch. He doesn't explain it fully, but tells Lloyd to remember his lineage. Suddenly, these rifts start appearing around Ninjago, first small, then big. Dragons start rapidly migrating through the realms, acting all wild and crazy. One of the rifts goes to Cloud Kingdom, where the ninja decide to go to gather some more info.
Enter Arin. He's a young writer of fate living in Cloud Kingdom, assigned to write menial happenings like people tripping or losing their socks. He doesn't hate his job, but he wishes he could write his own fate. Anyway, he sees the ninja asking the writers about coalescence, and the writers explain, rather coolly, that coalescence is caused by a shortage of aether, which is caused by the imbalance of good and evil energy that came as a result of destroying the Overlord. Aether is the stuff that lines each realm and makes up the space between them, so a shortage of the stuff would naturally cause them to collide. The ninja know that kind of problem is a little out of their pay grade, but they're stubborn as hell, so they insist that there must be something they can do. The writers simply say that fate is what it is.
The ninja try coming up with some kind of plan, and Arin decides that it's about time he did something for himself. He tells the team about a realm he's written about before, called the Realm of Wisdom where a swamp of supposedly infinite wisdom resides. They're willing to try anything, and that sounds mighty promising, so they tell him to go ahead and write them there. Arin, a bit embarrassed, explains that the writers of fate don't actually choose fate themselves, they just document what comes to them, which makes it happen. However, if he tries writing something that didn't come to him from fate, it doesn't work. He demonstrates this by attempting to write them into the Realm of Wisdom, to which the ink simply fades.
By this point the Cloud Kingdom writers have noticed one of their writers is missing. They're chased out of the kingdom, Arin following. Luckily, with all the rifts and such, they're able to find the Realm of Wisdom, popping into Ninjago to see that things have gotten worse. The Realm of Wisdom… Does not look like it. It seems the "swamp of infinite wisdom" is a literal swamp, inhabited by frog people who speak in weird riddles. They decide not to judge a book by its cover, however, and, after becoming acquainted with their peculiar culture, they ask one of the frogmen how to stop coalescence from happening. He tells them he will answer their question for a price. It's fairly simple, he just wants the ninja to collect him a baker's dozen of flies for him down at the Marsh of No Return. They ask him why it's called that, and he tells them that question comes with its own price.
So they go to the Marsh of No Return, very quickly learning why it's called that. It is a pit so deep, that they would never be able to get down there and back without it taking forever. Plus, there doesn't even seem to be any flies around. That is, until there very suddenly are flies around, and they're very big and angry. The ninja get, like, one or two, but the swarm leaves before they can get more, and the realm starts falling apart, so they really don't have time to wait. They go back with what they have and ask the frogman if there's anything else they can do. The frogman says yes. On queue, a strange man on a motorcycle falls in from one of the rifts. The frogman asks them to get rid of him.
They go see who this guy is, and they see he's welding this special bladed weapon that can rip holes directly into space time. That's not cool, so they go to stop him. There's an epic fight where the ninja find out this guy, Ras, wants the worlds to collide, because he believes the energy from all the realms will burst again to create an even better universe. That's all they can get from him, though, before he dips out. They still did technically get rid of him, though, so the frogman tells them the answers they seek are in the Realm of Madness. They're a bit upset since that's not really a full answer, but they're mostly scared since the Realm of Madness definitely lives up to its name.
They go through some other realms on the way, maybe even running into Ras again, and then they end up in the Realm of Madness. This realm, previously completely inaccessible by any means of travel, features only strong, raw elemental energies, and, as of late, many dragons. The elemental energies and dragons almost seem to be fighting. The ninja's powers start going kinda crazy, since they're around all this elemental energy, but, strangely, Arin is feeling weird too. Each of the ninja are approached by dragons of their respective elements, and they seem to remember the good they have done for them. Then, this tiny, funky dragon approaches Arin. He seems to like him. They have no idea what element it is, but soon find out when it unleashes its powered breath and fixes a small rift in the realm. It's an aether dragon.
Arin starts to freak out a little bit, since that could possibly mean he can control aether. Lloyd, having been in a very similar situation before, immediately takes on the role of Arin's mentor. Since they know elemental control is strongest in a place where there's a lot of an element, they go and try to find some aether. Unfortunately, though, Ras is back, and puts up quite the fight before essentially kicking them out of the realm back into Ninjago. Things are WAY worse now, and rifts to other realms have basically taken over the sky.
The rifts become unpredictable, constantly cycling which realms they lead to as they begin to collide. They don't have dragons, but a new, epic character can do them one better: a realm-hopping mech/motorcycle thing. This character, Sora, says she doesn't belong to any particular realm, but was part of a band of realm-hoppers that went as they pleased and did what they wanted. Arin is very interested by that idea, and feeling a bit conflicted. He likes the idea of being important to the fate of the realms, but it's a lot to handle, and it's not a fate he gets to choose. He confides in Lloyd about this at some point.
Anyway, Sora's whole crew got swallowed up by a rift, so she's really desperate to find them, but if the ninja claim they have the key to fixing this whole thing, she'll help them. They tell her they just need to get to some aether, and Sora isn't too sure how to do that, but has an idea. It's easier than ever to go through a lot of realms really really quickly, and there are still slivers of aether between them, so if they speed through a bunch, they might get close enough. This all leads to a really cool animation sequence where Arin manages to grab hold of a small piece of his element, and maintain it under his control. He's excited about it, but knows he has a long way to go.
Anyway, as Arin starts to build up his abilities, he really starts to bond with the whole ninja team. They all share stories with him about when they were first learning to control their powers, which helps him control his own. They end up confronting Ras again, and though Sora's bike is a huge help, they're only able to get away when Arin shoves him through a rift and closes it behind him with the power of aether.
With that, the team comes to a realization. Aether is created by there being a balance of good and evil energy, but those energies don't just exist, they're created by people. Ras created evil energy through attacking them that was contrasted with the ninja's good energy. Thus, Arin had enough of both to synthesize aether himself. Realizing what this means, they know that they need to find Ras again.
They go searching, and something terrible is starting. Coalescence. The realms aren't only connecting, but are becoming one. Places and people are collapsing into each other and abstracting. It's only a matter of time before everything comes together. They pass through Ninjago again, and see the monastery itself being affected by coalescence. Wu is barely hanging on, but he only tells Lloyd to remember who he is.
So now they're really on the clock. They find Ras again, but he's onto them. He will not fight them to give them the energy they need, despite their greatest efforts. He doesn't care about them anymore anyway, since coalescence is already here.
In a Hail Mary effort, Sora thinks maybe if they just fight each other, it'll create both energies. They try, but it doesn't really work. Lloyd is repeating Wu's line to himself over and over, and finally knows what to do. He decides to let his Oni form take over, though knowing that to truly create evil energy, he may have to let it take him more than he could come back from.
It works, and Arin, though still unrefined in his powers, of course, starts splitting things back apart. Ras tries to stop it, but it only creates more energy for him to pull from. In a great big flash of light, the audience is pulled back to the monastery, how it looked before all this.
It seems almost like it was all a dream at first, but surely not, because everyone remembers it and everyone is still there except for Sora, Ras, and Lloyd. Ninjago looks different now. Some of the realms can be seen in the distance with the naked eye, and people from all realms are walking the streets. As it turns out, Arin wasn't powerful enough to turn things back to how they were, but was able to bring back stability.
Everyone's mighty proud of Arin, and he is too, but he still feels like he never got the chance to do things on his own accord. Wu offers to train him to be just as capable as the ninja, to be the Master of Aether, but he respectfully declines, telling him he needs to choose his own fate.
That leaves the question of Lloyd. They call out for him, tell the city people that he's missing, and for a while, it almost seems as if he's gone. Then, suddenly, they see a flash of purple in the distance. Lloyd is most certainly not gone, and he's very mad. Oh boy.