The big reveals are the names of new Ultra Beasts, which played a big role in the original Sun and Moon games and appear to have an even larger role in Ultra Sun Ultra Moon
The other two known Ultra Beasts, UB Assembly and UB Burst, are officially named Stakataka and Blacephalon, respectively. Stakataka is a rock/steel Pokémon, while Blacephalon has an intriguing fire/ghost skill set.
It also appears that Zeraora, a new electric type, will be a mythical giveaway next year, based on mined data. Whenever Zeraora is made available — seemingly not for a few months after launch — it can be nabbed as a mystery gift or perhaps through a retail promotion.
Additionally, the legendary Pokémon Necrozma, who appears on the cover of both games, will have an Ultra form as well. In this form, Necrozma is reportedly one of the top 10 strongest Pokémon ever.
All of the data-mined information can be browsed in this Google Doc, but if you want to avoid story spoilers, beware. There are a healthy amount of data-mined plot points that may be worth avoiding for the sake of the surprise. The doc also has a list of confirmed Z Crystals and new moves and abilities. More information is likely to be added as we get closer to launch.
Pokémon Ultra Sun and Ultra Moon will max out the Nintendo 3DS power, according to a recent IGN interview with Game Freak director Shigeru Ohmori. For that reason, and probably also because the 3DS is in its twilight years, Pokémon Ultra Sun and Ultra Moon will be the last entries to appear on the Nintendo handheld. Fear not, though, since a mainline Pokémon game for Nintendo Switch was confirmed to be in development at E3 earlier in 2017.