Hi, I have a friend who works for Game Freak. They shared with me some information about an as of now free expansion that is planned to come to Pokémon Sword and Shield.

The expansion was meant to add a small part of the Kalos region, mega evolution and a reason as to why mega evolution doesn't exist in Galar. It has a small story that apparently isn’t much longer than an hour and a half, less if you rush. It begins with Sonia contacting you. While studying Dynamax and Gigantamax, she discovered a similar phenomenon where pokémon temporarily change into stronger forms, most oftenly happening in the Kalos region. She finds it odd that this doesn’t occur in Galar and has a theory that Dynamax and Gigantamax are linked to that. She sends you over to the Kalos region where a tournament is being held. You are tasked with gathering some data on Mega Evolution along with one of the Hoenn starters. You receive their respective mega stone, and a key stone. You eventually learn that the firing of the ultimate weapon is connected to the darkest day.

There was a hint of the addition of Mega Evolution in the Gotcha music video at around 49 seconds in when Calem stops and his mega bracelet lights up.