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Deeper exploration.
I mean a more immersive and dynamic world. Routes should be more than a straight line with 3 trainers waiting still that can be completed in less than 2 minutes.
Things like hidden grottos should return (and be expanded). Wild Pokémon should be seen running and flying while you travel (ORAS seems to do this already), like you can see roaming pokémon in the skies when they are near your area, fish pokes jumping from the water, digglets making holes around, purrloins running through streets in some cities...
Mountains and forests fully explorable and filled with the many items and monsters that exist in the Pokémon world, also lots of sidequests by hidden NPC.
Finding nests with pokémon eggs that you can steal, sometimes fighting with the mother if she discovers you. Things like that.
Just make the exploration be more complete and visually appealing. The story mode is mediocre and short, with a worse story every time. It's also gettting repetitive and more linear every gen. People just rush though it to start "competitive grinding game autism edition".