First and foremost, drastically reduce the plot or even outright remove it. I'm done with saving the multiverse from bumbling bad guys and engaging in endless cutscenes between characters I don't care about, just let me be alone and explore the world with my Pokemon, with my ultimate goal being nothing more than to become champion.
Dial back everything, no more ridiculous form changes or Megas/Z-moves, and drastically reduce the amount of items in the game. Either remove IVs or make them freely customizable in post-game without the need for chain breeding. EVs are fine but need to be more visible. What would be cool is if your Pokemon doesn't gain EVs right away, but instead you would have to go into a menu and manually apply the points yourself, just to prevent fucking up a mon's EV spread by accidentally fighting one too many Geodudes.
Continue with the trend of shrinking each new generation of Pokemon, minimize the amount of legendaries/mythicals a la Gen 6, but also throw us a handful of cross-gen evos, maybe 7 or 8 max, and get rid of Eviolite while we're at it.
Cut out every ridiculous side mode or gimmick, no more contests or Amie/Refresh, no playing with Pokemon on the overworld like USUM uselessly added. Put all of that development into battle facilities like Frontier or PWT, or expand the amount of options you have in multiplayer.
Rather than difficulty modes, introduce a "veteran mode" where all tutorials and cutscenes are skippable by default, and where NPCs don't shower you with free heals and items and convenient plot warping every three seconds. If casuals want that, fine, but don't condescend against players who know what they're doing.
Just as a little fun side thing, add a bunch of variable color palettes, determined by the Pokemon's nature, but also make them customizable after a certain point, so that you don't get stuck with a Pokemon having a hideous color scheme in its optimal nature, and so that you're less predictable in multiplayer.
