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Arceus itself says that wants to see if human's heart has changed and will eventually join the player later on, if battled and catched.
Some details: Dialga and Palkia will not have mega/primal, but will have a sort of Z move aesthetic change (like Lunala/Solgaleo)
Mega Giratina is more linked to the origin form than the altered one. Its total stat wil be 780, its ability Distortion and it will remind a bit of the Gravity move, but not only)
Primal Arceus ability will be called Virdict: its moves cannot fail and are always super effective.
Sinnoh will be explored in many different ways, trough time and space travel. Also there will be new parts of the region to visit in the South and in the West, as well in the sea.
Gastrodon will have more forms. There will be some new Mega and the variations presented in Let's Go 2 will be expanded to more Pokémon. Arceus space and the Distorted worrld will be explorable and there will be some Pokémon there, like Malamar or Aegislash.
Pokkén League Frontier:
All the Pokémon from the curren Pokkén will be there, but some will be changed a bit, like Decidueye Or Darkrai.
But there are some new ones: 6 from 8th gen, then Deoxys, Lopunny, Snorlax, Dusknoir, Gallade, Magmortar and Electivire, Heracross, Infernape, Emboar, Samurott, Sawk, Hawlucha, Bisharp, Golisopod, Tortunator, Tapu Koko, Tsareena, Blacephalon, Buzzwole, Kangaskhan, Nidoqueen, Feraligatr, Shiftry, Slaking, Ursaring, Darmanitan, Krookodile, Escavalier, Lurantis, Beartic.
There will also be a so called "little cup", which will feature "weaker" pokemon.
For the other Pokémon games the only thing that is known is that 9th region will be based off an European nation. Italy seems a good candidate honestly.
That is all, for now. I'll stay online for a few minutes, if you have fast questions.