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Next Pokemon game will be D/P remakes, coming in November 2021.
They'll be called Time Diamond and Space Pearl and will feature Dynamax/Gigantamax.
It will cover the same story as D/P/Pt with minor differences, much like gen 3 Remakes.
No Sinnoh forms, but many gen 4 pokemon will get gigantamax forms being: Torterra, Infernape, Empoleon, Staraptor, Floatzel, Garchomp, Spiritomb, Rotom (every form), Dialga, Palkia, Giratina, Arceus
Rotom will get a new form, electric-steel and will learn flash cannon. He will take the form of a motorbike (similar to the one in sword and shield) and will learn Meteor Beam.
After you beat the champion, there will be a post-game episode where you catch arceus, nothing too fancy as things look very rushed/lazy.
Distortion World won't return, you will catch Giratina in the Turnback Cave.
Other than that post game is the same.
HMs won't return, but rotom will not be in your dex/bike.