Niantic has a habit of releasing regional variants of Pokémon in Pokémon GO long before the region itself drops. For example, we have had Alolan Sandshrew, Vulpix, Marowak, Meowth, and more in Pokémon GO before even the release of Unova, essentially giving us a taste of a generation multiple years away from where we are now. We saw Galarian Weezing launch in Pokémon GO not too long after the release of Generation Eight in the main series games and now have Galarian Zigzagoon, Meowth, Ponyta, Fartfetch'd, and more. What we haven't seen yet, though, is the release of Galarian variants of Legendary Pokémon. With Niantic's release patterns, though, it's an almost sure bet that we will see these added to Pokémon GO before we get the whole generation. Personally? I think we'll get them this year.
Niantic is known for slow releases for Legendary Pokémon. Generally, we will see a species debut in raids for three weeks to a month. Then, there will be a long absence. Often, there is a return pre-Shiny release. Then, the species has it's Shiny released upon another return, which will often last a couple of weeks to a month. Then, later on, after multiple returns, it will be given an exclusive move. If a species has multiple Formes, Niantic will repeat this in Pokémon GO with each Forme.
There's a problem that the Kalos Legendaries pose, though. There simply aren't as many of them as other generations. Sure, we'll likely see variants of Zygarde in its different forms that Niantic can play out, but they're very likely going to struggle to stretch the release of three species over the course of the year or more it'll take for the full Kalos rollout. I believe that we will see at the very least the Galarian Birds — Galarian Articuno, Zapdos, and Moltres — this year. Personally, my theory is that they'll be this summer's main Legendaries. It'd work as great cross-promotion with the Pokémon TCG, which we already know Pokémon GO is doing with their upcoming Professor Willow Trainer Card. The TCG's June set Chilling Reign features the Galarian Birds heavily, and what better way to celebrate the new and the old during Pokémon's 25th anniversary than with this update on an old classic?
What I don't think, though, is that we'll see Regieleki and Regidrago added to Pokémon GO soon. They tie i to the Regi family of Legendaries but are indeed entirely new species. This seems less fair game than regional variants.