As of right now, Pokemon Go players can participate in a giveaway that will allow them to dress up their characters with Pokémon Ultra Sun and Ultra Moon inspired clothing. The Pokemon Company posted the details on their site.
Taking place as a preliminary event to the release of Pokémon Ultra Sun and Ultra Moon for the Nintendo 3DS, Pokemon Go players can grab these outfits at no cost. The girls will be able to sport a sun hat, sandals, shorts, and a flower tank top; Guys will be able to grab sneakers, shorts, a tank top, a bucket hat, and if you so desire it, leggings.
Pokemon Go has been used to promote locations as PokeStops as well as holidays with cosmetic Pikachus and modified spawn rates. This is the first time Pokemon Go is promoting an upcoming Pokemon game, however. It's odd that this promotion would happen for Ultra Sun and Ultra Moon but not the first of the same generation, Pokemon Sun and Moon.
As it currently stands, there are no officially announced plans to start incorporating the most recent generation of Pokemon into Pokemon. Having implemented the second generation not too long ago, that leaves them with 5 more generations to implement if they play to let players truly "catch 'em all".