There is no a special Pokemon community to talk about so I write it here :)

The plot will be : You play Mew, and the antagonist will be Mewtwo. You live in a vibrant wild world where you will be able to encounter EVERY Pokemon based on their habitat.

As you play Mew, your only capacity will be 'morphing'. You'll therefore be able to embody other Pokemons. Here how it'll work (on next comment) :

In comparison to 'Mystery Dungeon' 's series, Pkmns live 'like humans', they talks, have jobs, etc. When you befriend a Pkmn, you unlock his 'morphing' form. E.g. you befriend a Charizard, then it allows you to use morphing on him to copy him and become a Charizard.

You can befriend Pokemons by helping them, or simply talking to them (for basic Pokemon). Powerful Pokemons requires to accomplish (very) hard quest to befriend them (strong Pkmns like Garshomp, Zapdos, etc.)

In theory you can embody EVERY Pokemons. Usefulness of every incarnation : The goal is to remain faithful to the real abilities of the Pkmns (see next comment).

Water Pkmns: depends on their swimming ability, a Palafin will go very faster and have a different swimming trajectory than a Relicanth for example.

Flying Pkmns: like water Pkmns, they all fly but differently, some can fly longer, others glide, some will be special like Ho-Oh who could leave a rainbow trail behind him in the sky. Some of them can even swim and dive into water (like Cramoran, Lugia, ...)

I won't describe all the types, but I hope you can imagine that each Pkmn has a unique purpose and can be used to accomplish specific tasks. Hence the importance of acquiring as many morphing forms as possible.

Now let's talk about the plot, then we'll talk about the general atmosphere of the game and some scenes I have in mind that would work well in this universe and with this gameplay :

Mewtwo is corrupting Pokemons with the aim of destroying the world and separating them. He will be accompanied by a lot of other legendary Pokemons like Hoopa, Groudon, Genesect, Giratina, Eternatus...

All corrupted Pokemons will have red eyes, as if they're hypnotized by Mewtwo. Then as Mew, you'll have to gather Pkmns, ease tensions between them and calm the corrupted ones in order to decrease Mewtwo's strength and befriend with the 'pacified' Pokemons.

Mewtwo will be the final battle in which it'll be accompanied by other Legendaries. As you've guessed, you'll have to face him using morphing forms you've obtained... And now... let's imagine... (see next)

...Imagine a battle (I precise in a open world style) where you must face Groudon in its 'final land', shift dimensions to confront Ultra Beasts, experience an ice age brought by Kyurem's fury, be trapped in the depths of nightmares because of Darkrai...

...be plunged into the cosmos far away in the stars by Deoxys who wants to eliminate you. Explore the dark depths of the abyss where a gigantic eye opens before you and you discover that it's Kyogre, and the battle begins with an unexpected epic music.