Post stuff like form changes(like Shaymin/Giratina) or any variant that isn't temporary(Mega, Giga, Primal)
Pic related. I love the idea of Megas and I get they're too powerful to keep but some Pokemon really do need a buff and I love the idea of Gardevoir with a new dress(especially the shiny variant's black dress)
Would be a good excuse to reallocate some of the wasted stats or give a general buff.
Beedrill is another Pokemon I really wish had a permanent alternate form just because of how awesome its mega was. It looks so fucking good and speedy.