I’ve been able to gather some info about the SwSh DLC and the DP remakes, however not all is confirmed yet especially in the DP remakes as they are still quite far away. Unfortunately I was not able to get any info on any Let’s go games.
New Galarian forms from both DLC’s:
Galarian Jynx- Ice and Psychic
Galarian Arbok and Ekans- Poison and Fairy
Galarian Slowbro- Poison and Psychic
Galarian Slowking- Poison and Psychic
Galarian Dustox- Poison and Fairy
Galarian Phanpy and Donphan- Steel and Dark
The PC and GF are planning on more Galarian forms but these are the only ones I know of so far.
In the Crown Tundra DLC the legendary Calyrex has an alternate form in which the bulb on its head has blossomed into a beautiful flower. Also in the crown tundra you will be able to purchase mega stones for battle points in a small outpost on the side of a mountain.
DP remake news:
Mega evolutions will be in the game with all three starters getting a mega evolution. Other Pokemon including Luxray, Yanmega, Mamoswine, Floatzel, Gliscor and Weavile are all planned to receive mega evolutions.
The main story is the exact same as DP as they are remakes, not retellings, however there are two sections to the post game. In the first half you go on a journey to the distortion world to capture Giratina using the legendary Dialga/Palkia which you would have previously caught. In the second half of the post game you learn a lot more about Arceus and eventually you catch him, I don’t want to give away to much about the storyline of the postgames so I won’t give any more info on them. And Finally the last thing I was told is that if you hatch an egg of a Pokemon with a regional form in the distortion world it will hatch into that regional form however I don’t know how GF will deal with this when it comes to Pokemon that have multiple regional forms.