At the present time, there isn't much more information known about Pokemon Ultra Moon outside of the title and the Release Date of such. However, one thing we do know is that a number of Pokemon will be given a brand new form in the game, starting with none other than the Legendary Cover Mascots Lunala and Solgaleo (as pictured below).
These two Pokemon have new form appearances after somehow fusing together with the mysterious Necrozma - being dubbed by fans as their "Ultra Forms" - and it currently looks like this is done somehow with a player's Z-Bracelet they obtain during the game. From there, it is still unclear how Necrozma will play a role in the upcoming game and what type of stat increases Lunala and Solgaleo will have after merging with it.
In a later leak, it was shown that these forms are actually that of Necrozma after it literally absorbs either Solgaleo or Lunala. When Necrozma absorbs Solgaleo, it's called its Dusk Mane Form. When Necrozma absorbs Lunala, it's called its Dawn Wings Form.