Ultra Sun and Ultra Moon:
-Honestly, have Dulse and Zossie be out and about in both games and leave Phyco and Soliera to the boring office work back in Ultra Megalopolis. Dulse and Zossie's dynamic actually improved my experience playing the game, and I wish they'd gotten more screentime and better battles (or any battles at all, in Zossie's case. And speaking of Zossie, I kinda wish she had bonded more with the MC since they're the same age. That would have been adorable.)
-No Rainbow Rocket episode. Bring back the original Ultra Beast hunt mission, just improved and with added involvement from Dulse and Zossie as said above.
Let's Go:
-Bigger Pokédex! The game overall was very well made, but limiting the Dex to only the first 151 was a major misstep that FR/LG had already made before. Add all the cross-generational evolutions up to Gen VII (or hell, if they decide to do an updated version, the current generation), and maybe some Johto and beyond Mons that would fit the Kantonian landscape. Should be about 300-350 Mons.
-As an extension of the above, breeding and better teams for every major character.
-Have the regional forms of Kanto Mons be catchable, not just trades. That way, we can nickname them whatever we want.
-Have Meowth be able to inexplicably speak, like in the anime. I felt bad for him when Jessie scolded him at Mt. Moon and he couldn't just talk back to her.
-Character customization on par with the Kalos, Alola and Galar games or hell, even BD/SP. Now I see that the complaints about the Naranja/Uva Academy uniform in S/V go a longer way back than I thought...