I actually hate tall grass, tbh.
My idea of the "perfect" Pokémon game would be the simplicity and catching style of Pokémon Go, a beautiful and explorable overworld with great music like in XD Gale of Darkness, interaction with the environment like in XY (sitting, swinging on vines, climbing rocks), riding Pokémon with no HMs (like in Sun/Moon), and more realistically-scaled objects when you're "outside."
Things that would make it more fun (rather than necessarially a good game) would be a better balance of fully-evolved non-legendary Pokémon, balancing of the types (ground typing's offense and electric's defense are too good compared to others for instance), making legendaries feel significant rather than a wall that needs to be caught (this felt better in XD Gale of Darkness, for instance), make the trainers harder and late-game leveling less grindy. Make getting shinys way easier (hatching 2,000 eggs isn't very fun).