DISCLAIMER: This post will feature spoils for Pokémon Sun and Moon, and will talk about divergences between those games and the upcoming Pokémon games, Ultra Sun/Ultra Moon. This post will also contain some speculation based on footage from trailers for the game.
Pokémon Ultra Sun/Ultra Moon will release in November 17th, nearly one full year after Pokémon Sun/Moon. According to Pokémon tradition this would make the new games the 'third' game in the set.
Game Freak always releases the same game twice, with slightly different version exclusive Pokémon and almost no differences in the story. Two years later, they typically release a third game. Plotwise identical to the other two games, the third game usually has an extended post game and a few more features. This time appears to be a lot different though, and I'm going to talk about the way the game has changed.
Brand New Story
Based on trailers and interviews with Game Freak, it seems apparent the new games has a similar story to that of Pokémon Sun/Moon but diverges at some point when the Ultra Recon Squad arrives on the scene. It's not like Platinum or Emerald either, apparently. These are major changes.
Pokémon Sun/Moon was set in the Alola region and focused on the Island Trials, with a secondary focus on stopping The Aether Foundation and it's president Lusamine. Lusamine is a heartless woman and abusive parent who strives to unlock the power of the Ultra Beasts. In the post game Looker and Annabelle have you track down all the Ultra Beasts who have come through wormholes onto the islands.
In Ultra Sun/Ultra Moon things have changed. The trials have changed. Mina even has her own trial now. Travel between the islands is less linear, and is now a sort of mini-game in the form of Mantine Surf. On top of all that the region is shrouded in darkness. While the reasons behind the opening of the Ultra Wormhole in this game is not yet clear, the trailers definitely indicate that Ultra Space is a much larger part of the game. Instead of a hallway full of Nihilego, you can travel to multiple wormholes by riding Solgaleo or Lunala.
Each wormhole appears to be a different habitat that houses a different Ultra Beast. You travel across a electrical forest with light up roots and a bouncing Xurkitree, you can meet an angry Buzzwole standing in front of an active volcano. This is most apparent in footage from the trailer where Guzzlord and Xurkitree can be seen in cutscenes.
Moreover, many of the trailers put a fair amount of emphasis on the Ultra Recon Squad, a new group of characters who are trying to stop Necrozma from stealing the light from the Ultra Megalopolis. The Ultra Recon Squad seems to battle the player pretty earlier in the story, as evidenced by the fact that the player is using a Dartrix. The trailer shows the Ultra Recon Squad using UB Adhesive as one of their Pokémon, being commanded by Soliera in one shot, and Dulse in another.
Interestingly, it seems the members of the squad are version exclusive, with Dulse and Zossie in Pokémon Ultra Sun and Phycho and Soliera in Pokémon Ultra Moon. It's unclear what impact this difference has on the overall story of the games. More footage reveals Dulse and Zossie in Aether Paradise, and then later approaching the player with a Cosmog in an outdoor area. Phyco and Soliera seem to encounter the player in Iki Town, and in the Lush Jungle. However, these could be unseen areas that look like existing areas.
Brand New Pokémon
Another major change in the upcoming Pokémon game is the addition of three new Pokémon, and new forms of existing Pokémon. The three new Pokémon are Ultra Beasts who's proper names haven't been revealed yet.
UB Burst and UB Assembly are available in Ultra Sun and Ultra Moon, respectively. Meanwhile, UB Adhesive, like mentioned above, seems to be primarily used by the Ultra Recon Squad. Almost nothing is known about these new Ultra Beasts, apart from their typings and the fact that the are all Ultra Beasts. Some gamers hypotheses that UB Adhesive may evolve further. While three new Pokémon isn't a lot of new content it is still a big deal. Game Freak have never before added an entirely new Pokémon in the middle of a generation, let alone three. Apart from adding some new Pokémon, they have made some older, previously inaccessible, Pokémon available again. Pokémon Sun/Moon gave the player access to 330 Pokémon, several of them brand new, and several of them old. The newer trailer promises that over 400 Pokémon can be caught in the region, and showed off about 15 of them. That means over 55 Pokémon that we couldn't catch in Sun/Moon without the use of Pokémon Bank.
It is currently unknown if there will be any other new Ultra Beasts, Alolan forms, or Pokémon, but several new forms of existing Pokémon have been announced. These new forms include Dusk Lycanroc, which is a mixture of Lycanroc's two other forms, and Necrozma's two new forms. Pokémon also have gained new abilities they previously didn't have access to. Lycanroc and Komm-o get their own signature Z-Moves, with speculation that Mimikyu and Togedemaru might also receive signature Z-moves. Solgaleo and Lunala also get new signature Z-moves, each of which can be used by Necrozma's new forms.
Necrozma's Role in the New Games
Speaking of Necrozma, his role in the new games are much different that his role in Sun/Moon. In last year's titles, the player could catch Necrozma in the post game, but catching it doesn't trigger any story or event. It's just another Pokédex entry. In Ultra Sun/Ultra Moon Necrozma seems to the main antagonist, sucking the light away from another world known as Ultra Megalopolis and perhaps causing the mysterious dark clouds floating above the Alola Region.
In the clip from the latest trailer it appears that Necrozma is waiting for you when you open the wormhole into Ultra Space at the temple on Poni Island. Lunala seems to think protecting Lillie and the player is necessary, as it dives right into Necrozma, before it can do anything.
Furthermore, Necrozma can fuse with each Solgaleo and Lunala, very similar to how Kyruem behaved in Generation 5. This new fusion of Pokémon allows Necrozma to use each of the other Pokémon's new Z-Moves.
Each new Z-Move has the function of ignoring the opposing Pokémon's ability. This is invaluable in certain circumstances, for instance either of these Z-Moves will undoubtedly OHKO any Mimikyu immediately, going straight through it's disguise with a stab-boosted super-effective Z-move. These aren't the only new Z-Moves that have been announced for Ultra Sun/Ultra Moon but they are likely the most important Z-Moves.
Rotom's New Powers
Ultra Sun/Ultra Moon introduces Rotom Powers. This isn't an entirely new idea, it's pretty similar to O-powers. The list of confirmed Rotom powers is pretty long and these powers can be unlocked through befriending the Rotom Pokédex. Each power has a separate application. Several powers have been confirmed and they seem to be the exact same as the O-Powers from generation 6.
Roto Hatch allows the user to hatch Pokémon eggs more quickly. Roto Bargain probably allows you to sell high and buy low in shops. Roto Prize Money might be another new way to improve the prize money rates from trainer battles. Roto Exp. Points and Roto Friendship will make leveling and raising friendship easier. Roto Encounter helps you run into high-level wild Pokémon more often. Roto Stealth will likely help the player avoid wild Pokémon. Roto HP Restore can heal a trainer's Pokémon a little bit. It is unknown if the other O-Powers will return as Rotom powers, but it seems likely.
It seems that these powers are items in your bag that Rotom gives you when you use the 'Roto Lotto.' It is unclear how often you can use Rotto Lotto, but it might be once a day. Rotom has one more trick up it's sleeve. Rotom has it's own Z-Power, too. Rotom's Z-Power allows the trainer to use more than one Z-Move in a battle. Thankfully, like O-Powers and Pokémon Refresh, this intense powerup seems to be relegated to the main story, with the meta-game and battle facilities being more balanced.
Pokémon Ultra Sun/Ultra Moon will release November 17th.