No more HM slaves, it was a dumb feature that limited our teams to four-five Pokemon for too long. Just let us do what we did in LGPE and ride a Pokemon to get us around obstacles.
Following Pokemon.
Memory link, maybe have it let us rewatch cutscenes or see certain character backstories.
VS seeker, my favorite thing for training.
More customization, seriously we have people with blue, pink, green etc., hair but our character has normal hair colors and relatively normal hairstyles? I want the option of being an anime character with like five different streaks and I want to pick where I want these streaks and how big I want them to be.
I like Pokemon Amie/Refresh, not sure if I like the healing mechanic as that might make things too easy for other players but I want this to return and maybe even be revamped with toys, books, other Pokemon interactions, bed etc,. Maybe make it like a resort we teleport to via menu option.
Soaring
Something we can send our boxed mons to, I dunno about everyone else but I always felt bad when I'd box a Pokemon who I knew in story loved people, was a gift, or a legendary who's important to the world. That all changed with Poke Pelago, where I knew they'd have fun and I could put them to work without removing them from the box.