My personal thoughts right now, after that I'll respond to other's comments:
Ultra Wormhole Travel:
So confirmed is a full feature! Sweet! But could is be better than riding Rayquaza into space, Mega Evolving, and destroying a meteor?... Well, no, but what we're getting is still cool and more multi-functional!
For the travelling part, gotta say Lunala looks more natural though Solgaleo doesn't look awkward since it creates a "floor" underneath it.
Trying to piece together the mechanics this feature will have. In addition to the various portals leading to the Ultra Beasts homes, there's also rings of light you're collecting and balls of darkness you're avoiding. So how does this factor in? Does collecting the rings of light make the portals appear and the more you get the more that appear and leads to different worlds? Do you need to collect a certain number of the rings to travel into the portals and some require different amounts? What happens if you get a dark ball? Do you just lose an amount of/all the rings and have to start over? Can you get a "game over" and be sent back from the wormhole you entered or are you forced to enter a portal?
Ultra Space Worlds:
Ah, so this is why so little changed in Alola: they were MASSIVELY expanding Ultra Space! Probably should have thought of that.
Actually I should say Ultra SpaceS. Another "why didn't I think of that", Ultra Space isn't one world but rather a space that connects multiple worlds together. Each Ultra Beast came from its own world, and now we're getting to explore them and encounter the Ultra Beasts (we won't get into how odd they're the only inhabitants of their world)!
That said, from what I'm seeing they Ultra Beast home worlds are kinda looking linear. Looks like the areas are more just to provide a spectacle and Ultra Beast encounter than being a fully fleshed out area.
Also I'm wondering if we'll see the homes for Guzzlord and the new Ultra Beasts. I ask because we saw they'll be in Alola and the Ultra Beasts homes will be providing an encounter location. Of course they could just have us defeat the UBs who came to Alola which sends them back through their wormhole, thus giving a reason to include their home world.
Finally, what about the Cosmog family and Necrozma? Do they have a home world?
Ultra Recon Squad:
CALLED IT (at least in one of my theories)! Native people of an Ultra Space world! And they're pale blue faced people with purple, bright orange, and blue hair. Intersting, wonder if they're home having no light caused them to change to be like that.
One thing about them that caught my eye was that quick scene of them in Aether Paradise. It sort of looks like the beginning cutscene of Lillie escaping Aether Paradise, with them replacing the grunts. If that is the case, does this mean the story has been changed in a major way? They now have something to do with Lille running away?
Odd they're making which two members you get version exclusive, but then again that's no different than the Hoenn games where you're either dealing with Team Aqua or Team Magma. They'll all probably show up together in the end so you at least get to meet the other two, sort of like how ORAS had the other villain team admins appear at the Battle Resort for a quick cameo.
Nothing to say on the individual members. In Ultra Sun you get Dulse (serious young man/purple hair) and Zossie (little girl/orange hair) while in Ultra Moon you get Phyco (mustache man/blue hair) and Soliera (woman/blue hair). I want to say maybe the pairs have a famiy relation thing: Dulse and Zossie being brother and sister, Phyco and Soliera being father and daughter (Soliera's actions during battle seem a bit immature that makes me doubt something like husband and wife). It'll also keep in one of the themes of Alola being family.
Considering you're seen wearing one of their suits and the ones you encounter let you explore their home world they're not going to be villains, probably just people on a mission to get the light back in their world. Maybe they're also trying to prevent Necrozma from taking the light of the Pokemon World, OR maybe they're going to have Necrozma steal the Pokemon World's light than capture Necrozma to bring light back to their world (In the sneak peak trailer it looks like eventually the Ultra Recon Squad trap it on top of their tower and examining it).
But still, PEOPLE OF ULTRA SPACE! Okay, yes, technically Sun & Moon did give us the first non-Pokemon alien, but that was just a quick "what the" cutscene. This is the first game where you're battling an alien sentient species of human level who use their own world's Pokemon to battle!
Also they're battling theme sounds great! Other worldly but also sci-fi, showing how they're the human version of Ultra Beasts essentially.
New Ultra Beasts:
I go more into depth about my thoughts on the new Ultra Beasts in an upcoming JAPE. I'll also share my thoughts via other's comments.
Necrozma Main Villain:
So yeah, I think this is also a first: the mascot Pokemon is the main villain, at least for the main game series. While we've had the likes of Groudon/Kyogre and Giratina, they weren't villains but rather forces of nature released/angered by the villain team. But sounds like Necrozma is going to be a villain of sorts, wanting to absorb all light for whatever reason and having done so to the Ultra Recon Squad's home world! Though it seems like it needs Solgaleo or Lunala to do it, as the one in Sun & Moon not only just flew around at most but looked like it was in physical pain at times (maybe what we mistook it as trying to hold back its power was actually it reacting to not being able to absorb light for a long time?).
If I had to take a guess on how the story changed, the story now is more about stopping Necrozma from stealing the Pokemon World's light. Lusamine is prevented from entering Ultra Space, she still does her releasing Ultra Beasts thing but something stop her and maybe she snaps out of her insanity (apparently she warns someone about a shapeshifting monster, which sounds like Nihilego though I guess could also be Necrozma). Though Lusamine didn't enter Ultra Space, you still got to "cure" Nebby which is why you evolve it... which is then when Necrozma appears and possesses it. Your goal now is to rescue Nebby, which might involve having to get the other Cosmog and evolve it, can't see any other way to travel into Ultra Space unless they do a risy "one-way-in" way via Burnet's lab and/or Aether Foundation. Though they'd probably have to raise the stakes, like have Necrozma steal the Pokemon World's light, to justify the player do something that risky alone.