Hello. I am on Gamefreak's English translation team for Pokemon Ultra Sun and Ultra Moon and thought I would share a few details about the game.
>the legendary pokemon shown in the trailer are not new forms of Solgaleo and Lunala, but rather Necrozma's Radiant Sun and Full Moon forms. It transforms by using one of 2 exclusive Z-moves.
>some of the new alolan forms include an electric type Alolan Dodrio, a dragon type Alolan Ekans, and a rock type Alolan Lapras obtained by reviving a fossil.
>the story is mostly the same as Sun and Moon, though Lusamine seeks to unleash the Ultra Beasts by using Necrozma instead of Cosmog. There is also a mission after the beasts are unleashed where the player, the rivals, zygarde, the kahunas, and the tapu defend alola from the beasts. UB-05 Guzzlord and Zygarde have a special role though I dont know what it is. Cosmog is notably absent until postgame.
>gyms do not return, but some of the totems at the end of each trial are changed from pokemon sun and moon.
>some new postgame areas are added where you can catch legendaries from previous games, such as a volcano where Heatran appears. This does not include "mascot" legendaries such as Dialga or Xerneas.
>Move tutors are added through festival plaza shops. They require Festival Coins as payment.
>New trainers are added to the battle tree, such as Steven and the rival from Gold and Silver.

That's all I know. If I learn more I'll make another thread sometime in the future.