Hisui

Ingo returned home and the Battle Subway is once again operational.

Laventon’s original Pokedex was lost but was recently rediscovered in somebody’s private shed in Sinnoh.

Volo is somehow still alive and donated his Celestica ceremonial clothes to the Lumiose Museum’s Hisui exhibit.

When the Lumiose Museum asked Wikstrom about Kamado’s armor’s resemblance to his, he said it was probably something his ancestors gifted to Galarians long ago.

Rei and Akari's clothes are on display at the museum. Make of that what you will.

Kalos

Sycamore retired as director of his lab. (My headcanon is he blamed himself for not catching on to Lysandre sooner and resigned out of shame but that part isn’t stated).

Trevor and Sycamore are currently away in another region on research.

Tierno is in Unova working as a professional dancer, and seems popular.

Clemont is still making inventions but Lumiose Gym closed because of the renovations on Prism Tower.

Most of Team Flare is in prison, except Mable who got out on a plea deal. She is not allowed to leave Lumiose so the government can keep an eye on her.

There is a photograph in Emma’s office of Xerosic getting along with Mimi so he seems reformed.

Siebold has retired from the Elite Four to focus on his cooking.

Malva is missing/dropped out of public eye, probably to avoid the Team Flare fallout.

AZ’s Lucario was originally Korrina’s.

Diantha is still acting and has a new movie coming out.

Galar

Former champion Leon visited Lumiose only a few days before the game starts. He participated in one of Jacinthe’s tournaments and got lost while touring the city.

Endgame Spoilers under break

my gym leader's kinda homeless

my take on adult Clemont :3 when boyfailure turns to manfailure