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Bring back Contests, Musicals, The Pokeatholon, all the old Pokemon Stadium style mini games plus new ones you can play with your Pokemon.
Flesh out an actual alternative playstyle with actual progression, like the Hoenn contests but more. Actual programmed and designed challenges instead of RNG.
Also bring back Frontier style post game battle challenges that aren't just "100 battles that get increasingly harder to the point of cheating in item/move pokemon combinations that aren't actually in the game".
Bring back ball capsules and following Pokemon too. Give my balltism a point, and let your following Pokemon wear their ribbons so you can show off all those grinds for contests/battle tower/etc.
Basically, just put back and/or expand on the little distractions that add novelty and completionist aspects, because if all I gave a shit about was mindlessly sweeping with some busted build I'd play showdown instead.