Two more videos have been released, these expand on the mysterious new characters...
[yt]https://youtu.be/bucxOT8zudk[/yt]
[yt]https://youtu.be/1p8nBXLQMPs[/yt]
[spoiler]
Now you can ride on Solgaleo and Lunala to play some sort of Ultra Wormhole minigame to enter various kinds of Ultra Spaces, and battle the various Ultra Beasts within. Encountered ones include Nihilego, Buzzwole, Pheromosa and Xurkitree.
Apparently you will enter an alternate dimension, in it there's a city called Ultra Megalopolis, where Nercozma has stolen the light. This is the headquarters of the Ultra Recon Squad, the four mysterious characters that was revealed in a poster a few weeks back. They are named Dulse, Zossie, Soliera, and Phyco. You battle 2 of 4 in each game (Dulse and Zossie in Ultra Sun, Soliera and Phyco in Ultra Moon) and they seem to battle using a new Ultra Beast, UB Adhesive.
UB Adhesive is a pure Poison-type, with Beast Boost. UB Burst is revealed to be Fire/Ghost, with Beast Boost and a new move called Mind Blown. It deals Fire-type or Ghost-type damage, but UB Burst receives heavy recoil damage. UB Assembly is Rock/Steel with Beast Boost, and knows Iron Defense and Wide Guard.
[/spoiler]