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Go back to the Japan regions. A mix between Kanto and Johto with more areas to the north. Actually flesh these areas out so they're more like a real-world and not a minecraft map. Add in Pokemon from other gens and place them so they fit in the world naturally. Have regular forms of new evos. Like a regular Farfetch'd more similar to his beta design or a regular Obstagoon. Have lots of little nooks and crannies to explore and discover secrets. Doesn't have to be open world but should let you make some choices like order of 1 or 2 gyms or take a harder route. Day, night and weekly events are back plus bring back seasons to make it more like a real world.
For gameplay mechanics: No gimmicks. Instead focus on more ways to actually further the gameplay. Like giving more options to the player and adding like certain moves can combo together for different effects. Nothing too drastic but feels like it could still work. Throw in nice little details and some fun minigames/postgame and you've got the perfect Pokemon game for me.