During the development of Sword and Shield's Expansion Pass, the idea of a third DLC involving the Kalos Region was in the planning stages, in a weird limbo phase between the drawing board and testing. I'll try and recount what I remember here.

The working title, "From A to Z," would have involved the character AZ in his attempt to harness Galar Particles to power a weapon that would protect the Kalos Region from an impending threat. Zygarde or a brand new Ultra Beast were considered to fill that role, but this was never finalized.

Only parts of Kalos would have been accessible, with no option to challenge Gyms or the Elite Four. It would have contained a mix of exploration and Raid Battles, ending in an event that was probably a bit too ambitious for Game Freak to deliver. The idea was to have one huge Raid Battle as a live event that everyone could take part in simultaneously. It reminds me of what Fortnite has done with its concerts or Call of Duty with announcing their next title in Warzone. This would have been against the evil Legendary and culminated in a cutscene teasing Brilliant Diamond and Shining Pearl.

The cutscene would have featured Cyrus emerging from a portal to congratulate AZ. They would have escorted the weapon and escaped through the portal together, before revealing that "AZ" was merely a disguise. This seemed to have implications of a parallel timeline, but I can't be sure. There's probably a few other details I'm leaving out, but it's a shame we didn't get a chance to see this play out as envisioned. It would have changed the future of Pokemon reveals forever and given us a well-deserved return to Kalos!