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I got some information from GF, and I'll be as honest as possible with you.
No stupid information.
(So maybe I break your hype)
All I want from you is a little patience to read it.

Before we begin, I want to explain how I got this information.
I am a marketing worker who has been called by GF to perform certain work.

let's start
There is a good and bad news for you
Starting with the bad news, I'll list some things that we'll not have, and mechanics that definitely will not come back
> The Pokédex will not be so big
> The Megas were forgotten (but not discarded, I'll explain later)
> Z-moves has disappeared and will never come back
> Ultra beasts also not (forget them)
> We will not have Pokémon followers (if we have Pkmn followers, it will be Safari only, like "letsGO")
> And regional-forms. GF really had new region-forms ideas for Galar, they would be called "Galian-forms," but I believe they have been discarded. So don't have too many hopes about it.

The good news is that GF is working on a huge Feedback campaign on Sword & Shield, I'll explain.
If you want some old mechanics to go back to the next games (like Pokemon followers), please ask for it, high Feedback = GF listening to you

about the supposed images and videos that have been appearing on Internet last weeks
> All of them are fake, do not believe in any image of supposed pokémon that left before E3 and didn't come from an official source or CoroCoro.
> Next trailer will come on the final weeks of April, close to 20th probably (not sure, I might be wrong)

About Mega evolutions
> You must have probably heard that GF registered new brands of Mega-Evolution. The truth is that idea was not thrown away
> We have few designs of new megas, so probably, GF has put new Megas in this game, keep believing they will be back soon.
> Mega-Evolutions will definitely not be the focus of this game. Do not expect the Pokémon that you want to receive a Mega, will really receive it.