Here's what I have seen from the isle of armor so far.
Galarian forms(in isle of armor):
Dustox: Poison/Flying
Beautifly: Psychic/Bug
Smoochum/Jynx: Water/Fairy
Jynx get's a new evo. It's also water fairy, and it's a mermaid with Galarian Corsola shells on it.
Phanphy: Rock/Fighting.
Phanphy gets a new evo, it's rock/fighting.
Slowbro: Psychic/Poison
Aipom/Ambipom: Fighting
Seel/Dewgong: Ice/Fairy
Blitzle and Zebtrika also get one, but I don't know their type.
Deerling/Sawsbuck: Fire
Seviper: Psychic
Zangoose: Poison
There may be more but I don't know.

Gmax forms:
Galarian Dewgong, Jynx's evo, Phanphy's evo, Galarian Slowbro, Galarian Ambipom, Galarian Zebstrika, Galarian Sawsbuck, Kingdra, Starmie, Rotom, Salamance, Metagross, Palossand, Alakazam, Mimikyu, Falinks, Boltund, Ampharos, and Lycanroc (all three forms share one). There may be more, but that's all I've seen.

Story for the Isle of Armor mainly revolves around training Kubfu, and there is also a new evil team (don't know the name), that is causing disturbances around the Isle of Armor, and you have to stop them.

Kurt is on the Isle of Armor to make apricorn balls.
There is an appearance from Wake, the water gym leader from Sinnoh. I also heard Professor Rowan makes an appearance in the Crown Tundra, but I don't know for sure. The dlc is suppost to have many links to Sinnoh, to tie in to next years Sinnoh remakes.

That's all I got now. Not a direct, but a long trailer coming early June.