Now, this is getting pretty deep into baseless speculation, but I think it's interesting.
Have you heard about the Archeus and the Four Ethers theory?
Some things to read/watch:
The Archeus is the foundation of the universe and life. Kind of like quintessence.
The Four Ethers are four substances which makes up the Archeus. Think of these like protons, neutrons, and electrons to the Archeus' atom. These are: Life, Light, Chemical, and Reflective.
Basically, it's the idea that all the gimmicks we've been getting since XY are related to some of the Super Pokemon AND to Arceus itself. Most of these also just so happen to be Dragon-type Pokemon. AND they all seem to have some connection to other dimensions or other times. They all exist OUTSIDE the "main" Pokemon World.
So, it would break down to be:
Light-Ultra Necrozma-Z Crystals-Dragon/Psychic
Chemical-Eternatus Eternamax-Dynamax Energy-Dragon/Poison
Reflective-Terapagos-Terastral Energy-Unknown but possibly Dragon/Something.
And then there's Mega Evolution. There's not really a single Pokemon that's the "source" of that gimmick in the same way that the other gimmicks do, but there's some decent candidates:
A New Pokemon-I find this unlikely since Gamefreak doesn't seem to like Mega Evolution. I doubt they'd introduce a new Legendary that's the source of Megas, but it would probably work the best if they do.
Mega Rayquaza-Dragon/Flying. And like the others, it uses the method to change forms. It's also the most powerful of the possibilities. It's also the only one that has a Mega Stone as a part of it's body. The only problem is...it's not the source like the others. And there's NO connection to other dimensions.
Complete Zygarde-Dragon/Ground. Another strong possibility. And it has a hexagon pattern that's important due to the Terastral energy this generation. In theory, it should also have a lot of Infinity Energy, the energy that powered the Kalos Ultimate Weapon that...may have started this entire saga. Again, though, no tie to alternate dimensions...or even Mega Evolution, directly. I think it's more likely than Xerneas/Yveltal, unless they get a new form, despite them powering the Kalos Ultimate Weapon.
AZ's Floette-This one's kind of interesting. Some people are speculating that AZ's Floette...is actually a Paradox Pokemon. It has a "title", The Eternal Flower, like other Paradoxes like Iron Treads or Great Tusk. And it's possible that instead of resurrecting HIS Floette, AZ actually brought a Floette from alternate reality that never died, instead. The Kalos Ultimate Weapon is designed to look like a Floette. Also, AZ...Area Zero? There might be a connection here. But...except for probably having a lot of Infinity Energy, it's not really connected to Megas at all. Also, not a dragon.
Deoxys-I just find it interesting that the Kalos Ultimate Weapon MIGHT have drawn it here. I COULD see a new form of Deoxys that ties it closer to Megas. And according to ORAS, Mega Stones sometimes fall from outer space. And Deoxys came inside the Great Asteroid Delta. So...there COULD be a connection, here. Especially with that crystal in the middle of his chest, and it does change form by touching a rock. Again, not a dragon and no tie to alternate realities...but there IS a connection to outer space, which might be close enough.
But, it's also possible Mega Stones...aren't actually connected to this Fourth Ether at all. It's possible we'll have to wait until Gen X to see the Life Ether. Or maybe this is all completely wrong and it's not what they're doing at all.
Or it's possible we've actually been working our way through the "Four Worlds" instead.
en.wikipedia.org
XY would be Asiyah, with Zygarde concerning itself with protecting the physcial world. SM would be Yetzirah, with Necrozma (a "angel") looking and studying the world Arceus has created from outside of it. SWSH would be Beri'ah, with Eternatus being the "hand" that created things out of nothing, existing in the quintessence outside of reality. SV would be Atziluth, where Terapagos turns energy into physical form. So, then Gen X might be Adam Kadmon, where we MIGHT see the true form of Arceus.
Or...Could all four actually be enemies of Arceus? Maybe Arceus is actually basically one of many "gods". Perhaps even...a rogue "Legendary" Ultra Beast. We saw in USUM, Ultra Beasts either all come from alternate realities...or another possibility is that Ultra Beasts all "infect" and take over different worlds in order to reproduce. Basically, Ultra Beasts might be cosmic space viruses that use entire WORLDS to make more of themselves. Like how a virus infects and explodes out of a cell in order to reproduce. That MIGHT be what we saw Guzzlord in the process of doing on a alternate Alola.
So, what if Arceus is the only Ultra Beast who DOESN'T do that? What if Arceus is "reproducing"...by creating other Pokemon? What if the God of the Pokemon World is actually...a mutant Ultra Beast? And what if these Fallers that Arceus may or may not be sending to alternate universe...are actually being sent to those universe to "strengthen" them to prevent a invasion from outside the Pokemon Multiverse? Like how the player in Legends Arceus helps to create more Pokemon Trainers by teaching humans to trust and work with Pokemon...and therefore more strong warriors (or "white blood cells") that can fight back a potential Ultra Beast invasion?
Gen X could see a "Kirby Star Allies"-style LORE drop that kind of explains what all this multiverse stuff actually means.
