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This one's a bit unrealistic, in fact it would not work for many pokemon, but it would kind of make sense for some of them. Also I can't see this happening if not in a few years.
Pokemon that grow naturally, instead of "lv 40, *pop* you're 7 feet taller now".
For example, take a Litleo. As it gains experience, it grows as a lion cub would, getting first just slightly bigger, then lankier and more muscular, slowly taking a more Pyroar form. Then, at a certain level, it gets some red scruff around the neck or on top of its head, that, through the next two or three level grows into a mane, finally, at lv 35, officially turning into an adult Pyroar (and so getting the corresponding boost in stats and so on).
No fucking way GF would put that much effort in a gimmick, but one can dream.