1) Go back to 2D, a la the DS games. 2D sprites for characters (people and Pokémon), 2.5D overworlds, with animated sprites.
2) Revert pretty much all the nerfs - give Steel its Ghost and Dark resistances back, revert the special-move nerf (how 95 -> 90, 120 -> 110, and 140 -> 130 for moves like Surf, Thunder, and Overheat), remove the max-HP requirement for Gale Wings, etc.
3) Instead of nerfing OP Pokémon, give underpowered 'mon major boosts, like ROM hacks do - stat boosts, typing changes, ability changes, new moves, the works.
4) Have an actual goddamn postgame, like DS and earlier games did - BW had the eastern side of Unova, BW2 had the southern side along with PWT and Black Tower/White Treehollow and meeting N in the ruins of his castle. HGSS had Kanto and Red and Battle Frontier, Platinum had the Fight Area and Battle Frontier. FRLG had the last few Sevii Islands, while Emerald had its own Frontier along with the Steven fight.
5) Remove mandatory EXP Share mechanic, and make it toggleable like in the 3DS entries.
6) Revert Dexit and bring back every dropped move; they're all basically coded already, the moves, just flagged as 'unusable' and without animations. Heck, SV even had practically all the Pokémon species coded in, with actual stats and abilities - just lacking models (and thus they default to Pikachu).
7) Make the rival a fucking jerk again, to make beating him feel good - like the jackass deserved it. With these 'nice' rivals, they not only feel bland, but it's honestly quite pathetic how you continuously stomp them time and time again without them becoming spiteful; without a single time where you're supposed to lose to the rival, in order to give him a confidence boost or show that the rivalry is earned and deserved (that you two are equals).
Either make the rival a jackass, or make it more equal with a nice rival.
8) No more breakable TMs - we got this right on the DS, so why reverse this on the Switch? One-time-use TMs in BDSP and SV, and TRs in SwSh, force the player to grind just to get more, which isn't fun at all. In Gens 5-7 (and SwSh's TMs), infinite-use Technical discs allow for far more flexibility, as there's no cost to rearranging movesets on the fly (aside from the need to visit the move reminder, I suppose, for level-up moves and deleted TR moves).
9) One thing LGPE did well, for once, was to allow Repels to affect wandering Pokémon (the ones visible on the overworld), which was reversed in SwSh for some reason, which just led to immense frustration. If wanderers are going to be kept, instead of going back to wild encounters, then at least give us the option to turn them off with Repels.
10) Make villain teams VILLAINOUS again. Team Rocket's iconic not just because they were first, but because of how unambiguously evil they were, selling Pokémon off at the casino and caging them, not to mention the takeover of Silph Co., a major company. Team Magma/Aqua were going to destroy the world by awakening a Legendary, with the leaders realising their massive mistake far too late. Most of Team Galactic were goons, but Cyrus' plot still had weight; he had the pixie trio captured and tortured to obtain the gems needed to create the Red Chain, then shackled Dialga and Palkia at Spear Pillar in order to remake the world.
Team Plasma had a hidden dark side in BW, along with a likeable figurehead in N and the mysterious Shadow Triad; in BW2, they had a flying boat and control over Kyurem, freezing Opelucid and the nearby routes, not to mention the conflict between BW Plasma and BW2 Plasma members (those that honestly thought they were doing good, trying to separate Pokémon from people, only to repent upon realising they were the villains; and those that just wanted to be villains all along).
Even Team Skull was memorable, due to their weird dance, due to Guzma, due to their rundown home base. Sure, they weren't necessarily "evil", being a bunch of delinquents, but they were still villainous and needed to be stopped; from memory, they abducted a Pokémon, which is why you had to infiltrate their base in the first place. Guzma also had a connection to the Aether Foundation, to Lusamine, the actual villainess.
Meanwhile, Team Flare's forgettable, with the grunts being even worse than Galactic's and Lysandre's plot being nonsensical; Team Yell is just a bunch of rabid Marnie supporters (really? Her? Simps); and Team Star lacks presence outside of their separate storyline, and aren't even "evil" either (just rebelling against the school).