Earlier this year, Niantic's fourth-anniversary poster for Pokémon GO featured the Kalos starter Pokémon: the Grass-type Chespin, the Fire-type Fennekin, and the Water-type Froakie. The inclusion of content in these anniversary posters has historically meant that the Pokémon featured on the poster will be released at some point during the calendar year in which the image is released. This essentially confirmed that the Kalos Generation would begin its rollout during 2020, even though there are still many Unova species that have yet to be released in Pokémon GO. Generations are introduced into the game in waves, while new Legendaries often debut in raids soon in the overall rollout. This means that Kalos Legendaries could be entering Pokémon GO soon, so let's take a look at the Unova Legendary Pokémon that have yet to be featured in raids.
While every species of Unova Legendary has been introduced in raids, there are variations of their Formes that remain unreleased. Just like with Giratina Altered/Origin Forme, and the various Formes of Deoxys, it is likely that these special versions of these Legendary Pokémon will receive their own raid rotation rather than allowing for an in-game transformation.
The Unova Legendaries, known as the "Forces of Nature" are Thundurus, Tornadus, and Landorus. Their standard Formes, known as their "Incarnate Formes" which appear as humanoid genies, were introduced into raids earlier this year. While these raids weren't a fan-favorite, each of these Pokémon has another unreleased Forme that is far more dynamic in design: their Therian Formes, which depicts these Legendaries as more animalistic.
Then, Kyurem has two unreleased variations that increase its power to an unbelievable degree: Black Kyurem and White Kyurem. In the original games, Kyurem would fuse to either Reshiram or Zekrom to create these variations through "Absofusion," a process unique to Kyurem. It will be interesting to see how Niantic approaches this, as Mega Evolution sets a precedent for Forme changes… but it is unlikely that Absofusion will be introduced into Pokémon GO, as these two variations of Kyurem are already in the game's code as separate Pokémon.