I would prefer to stay positive, but there are a few things that kinda bugged me in the last 2 (3d) gens:
1) I'm bugged by the full-3d environements with no possibility of controlling the camera in any way.
2) Provide control of the battle camera please; the camera moves so much during battle for no perceptible reason (admitedly, this happened mostly when I was mildly distracted, trying to farm hordes/sos battles for shinies. It could take a good 30 seconds before I got a good view of ALL the pokemon).
3) Return of the national dex? The reasons I heard for no national dex went from "not important" to "3d pokemon models take too much space". Which is why most of my living dex has stayed in Gen 6.
Personally, I am a fan of graphical simplicity; games tend to age better that way (like LoZ: wind waker). I understand the desire for greater graphical fidelity, but it's never sold me on something. Maybe a more stylized visual approach could allow the 3d models used to look less awfull, without needing to cut interesting content.
Another thing I've always wanted is a return to something like Pokemon Ranch. Maybe a weird combo of a life sim (something akin to animal crossing, harvest moon, etc.) + pokemon, where you could build greenhouses, training halls, forests and pools for your pokemon to live (rather than rot in a box). I always love building homes in RPGs, and I loved the secret bases from RSE and ORAS. The Pokepelago had some elements of this, but I would love to see more. Anyways, that idea is more aspirational; not necessary, but I would love to see it.