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I had this feature in mind because I prefer some earlier evolutions compared to the fully evolved evos. So you go to this mystic person living in a hut on some route and if you bring him some items he will magically make your Pokemon look like a previous evolution but with the stats and movepool of the evolved Pokemon. The Pokemon also gets a special mark next to its name so you can't trick people online with it. Obviously this kind of worked better in my head without Mega evolutions being a thing, and different forms of Pokemon.
