I don't have much information, but here's what I know so far.
Galarian forms:
Dustox(Posion/Flying)
Beautifly(Bug/Psychic)
Poochyena/Mightyena(Fairy/Fighting)
Slowbro(Poision/Psychic)
Blitzle/Zebstrika(Water)
Abra/Kadabra/Alakazam(Ghost/Poison)

New regis are Regidrake and Regithunder.
At the end of the Isle of Armor, Professor Oak comes on a ship, and gives you an item that lets any Pokemon that can Gmax, get the ability to if they don't already. He also gifts you with a Bulbasuar or a Squirtle that can Gmax once fully evolved.

At the end of the Crown Tundra, Professor Rowan appears and says he is heading back to Sinnoh after 4 years of being gone(explains where he went for 4 years at the beginning of the Sinnoh games). This is a tie in to next years Sinnoh remakes (Temporal Diamond and Spatial Pearl).

Regigigas will not be in the dlc, as you have to transfer the 5 regis from the crown tundra over to the Sinnoh remakes to get him next year.

That's all I have now but if I get more I'll let you know.