It'd be cool if they brought back some older things to the games I liked the mini game like things (Pokeathelon, bug catching contests, ruins of alph puzzle sliding tiles, old game corner), Hoenn contests/berry system (honestly loved making pokeblocks), the three rods and fishing anywhere maybe just have the bubbling ones for special catches if done at all, optional follower pokemon, naming your rival, apricorns and Kurt, non talking pokedex it feels like Zelda from Spirit tracks or Navi they and it's annoying would rather the dex set up from Alpha and Omega, NPC trades (Idk why but I loved doing them), Run toggle, department store, secret bases, Treasure beach (Was fun to dowse there everyday, more lore about the region I liked having a good amount of lore that's intractable with, Marage spots like alpha and omega it was fun to fly around battle in the sky and find the places everyday, Honey/headbutt trees, hidden grottos, Field moves that get you items, Survival island from Diamond,
Keep the idea level up isle with a Nursery
New fossil pokemon, name rater like reborn, unique places for the catchable legendaires/mythicals like Birth Island, distortion world or Mysterious dungeon where once you catch the said legend pokemon fill into the place
From TCG Delta pokemon which are kinda like the alola forms, possibly some of the moves
Thanks for removing my double post @Ice Cream Sand Witch