- Real Leaks
I will mostly confirm and debunk and clarify on some of the most-talked about rumors, since I am less likely to get caught that way.
I will also attempt to leak some new information I believe to be safe.

I'm not using a tripcode. I don't even want to be tempted to reveal my identity.
I understand if you don't believe me, I only ask that you hear me out.

I can confirm that it is Kanto 100 years after Gen 1.
It is much different from the Kanto you remember. There's a mixture of new Pokemon and returning Pokemon.
Returning Gen 1 Pokemon have new designs and even typings in some cases. Some Alolan forms return instead of the original.

There are nostalgic flashbacks to the old Kanto hidden throughout the world. It's a little like Zelda's Memories in BOTW.
They are hard to find, but they open up small zones where you can catch the original versions of returning Pokemon.

Overall there is a lot less emphasis on plot than in the previous games.
The new team (not gonna say the name) is incompetent but wealthy.
They are responsible for a disaster that changed the region.

I don't know where the Johto and Unova rumors came from.
If these regions are in the game, no one in the NA localization team knows about it.
can confirm that there will be 2 versions: Pikachu and Eevee.
In addition to choosing a traditional fire/water/grass starters, Professor Mulberry will also give you a Pikachu or an Eevee depending on the version of the game you are playing.

Following Pokemon is back, kind of... If you're playing Pikachu version, then Pikachu will follow you.
If you're playing Eevee version, then Eevee follows you.

When Eevee evolves, the Eeveelution will follow you (and yes there is a new one).
Pikachu doesn't evolve, but its stats scale differently than a Pikachu caught in the wild.
Follower Pikachu is comparable to a fully-evolved starter by the time it reaches level 40.

THE SCREENSHOT with the Lapras and following Eevee is a total fake, the real graphics are a small-to-medium step up from that and the overworld camera angle is different from the 3DS games.
However, it hints at a lot of real info. It's possible that someone else on the team made that screenshot to leak without getting caught?
If so, it worked!

ULTRA SHINY is not a thing, as far as I know.
If there have been any mechanic changes to shinies, it is not mentioned in any of the scripts they sent over for localization.

Gyms are back. Poke Ride is back, there's no HMs.

The rumor about a Snake legendary is FALSE. I have not seen one.
This game's Mewtwo/Arceus is a very cool Poison-type Spider Pokemon.
I wish I could say more but I've already said too much.