Firstly, go watch this trailer if you haven't yet. https://youtu.be/ue-CBOrLnb0
Up to speed? Cool. WOW. I'm already in love with the idea of more Alola adventure!
But what does this mean? Those who haven't played or beaten Sun and Moon should probably stop reading because massive spoilers ahead.
During the events of Sun and Moon, we learn of the Ultra Wormholes, Ultra Beasts and Ultra Space. We also confirm that the Multi-Verse definitely exists by taking Solgaleo/Lunala to Alter of the Sunne/Moone and entering the Wormhole there, emerging in Alternate Alola.
The Multi-Verse was slightly teased during the Delta Episode of Omega Ruby/Alpha Sapphire, signifying that Pokémon X and Y and all games featuring Mega Evolutions are taking place in an alternate Pokémon World.
Anyway, back on course, with these things confirmed, what is the takeaway for the really small announcement?
Mr. Masuda mentioned that US/UM is an alternate story that takes place specifically in the world of Sun and Moon. Alternate and World are obviously the most important words here. He also mentioned that these games will include Pokémon that didn't originally appear in Sun/Moon. This could mean a slew of previous generation Pokémon could be available, but the focus should be that as he mentioned this, we're shown "Ultra Solgaleo" and "Ultra Lunala" who look like badass Zords.
What really strikes me about these Pokémon is that they seem rather familiar... not because of who they are but because of what their new form includes. Black prism like figures. Ringing any bells? It would appear that Solgaleo/Lunala have merged with Necrozma. 2 of these Pokémon are definitely confirmed to be Ultra Beasts while Necrozma is said to give off the same type of... aura, I guess.
To tie this all together, we could be experiencing one of two things: a journey to the Alternate World first shown in Sun/Moon with more story significance or just Necrozma merging with our Solgaleo/Lunala during the story to fight the final boss and potentially other Ultra Beasts. If the first is true, we could be fighting the final boss to stop her from taking over more than just Ultra Space. The 2nd scenario would feel like Crystal/Emerald/Platinum to me. Who am I to say though? This is only the first trailer and it wasn't even like 30 seconds?
The last clip also showed the player character with presumably a new Z-Crystal which could hint that the new Ultra form is triggered by using such. This isn't particularly relevant to my point above, but definitely worth noting as a cop-out to some really good story potential.
Regardless, I hope you're even more excited for Nintendo's presence at E3 and I can't wait to learn more about Ultra Sun and Ultra Moon!