One of the final things told of in the Famitsu Magazine Interview with a Pokemon Representative - and later confirmed by the Pokemon Company themselves - was the feature of being able to find Legendary Pokemon of previous Generations while traveling through Wormholes. This is reminiscent to how players could find Legendary Pokemon by soaring through the skies of the Hoenn Region back in Pokemon Omega Ruby and Alpha Sapphire.
Within the November 2, 2017 trailer that was released, it was told that every single Legendary Pokemon (not including Mythical Pokemon) could be found and obtained "just beyond" certain Ultra Wormholes. Some Legendary Pokemon will require special things in order to be found through a Wormhole: Girtina (Dialga & Palkia in Party), Suicune (Entei & Raikou in Party), Rayquaza (Groudon & Kyogre in Party), Kyurem (Zekrom & Reshiram in Party), Landorus (Tornadus & Thundurus in Party). It was also told that Ho-Oh, Raikou, Groudon, Latios, Dialga, Heatran Reshiram, Tornadus and Xerneas will be exclusive of Ultra Sun; Ultra Moon will be exclusive of Lugia, Entei, Kyogre, Latias, Palkia, Regigigas, Zekrom, Thudurus and Yveltal. Again, most of this is very similar to what players saw back in Pokemon Omega Ruby & Alpha Sapphire as far as the Legendary Encounters were concerned.