First, only normal and shiny Pokémon affect the texture of their form change (except eiscue), which sucks, because I really want to see what a pink Mega-Beedrill would look like. Or a Mega-Zombie Gyrados. Just adding textures for alt-Pokémon transformations would b enough to make me happy, personally.
Second, the main thing I would like to see (dunno if its possible) is if the transformation animations for Pokemon during and outside of battle could have more pizzazz and uhmph to them. Currently, having them flash white with an explosion sound and then having their model replaced with their alternate form is okay. It get's the job done. However, for each type of transformation, there could be something better to make the transformations feel impactful. Such as:
Megas/Primals: A crystal forms around the Mon' with crystal particle affects shooting out in rings, then once the model is swapped out the crystal is smashed open with a bunch of broken glass shooting away. The crystals could look like how they did in gens 6 and 7.
Wishiwashi/Zygarde collectives: A tornado(?) of colored objects swirls around the Mon', then once the model is swapped out they blast away from the creature.
Eiscue could have its ice head be broken like how an ice block is broken, and when its ice is grown back a bunch of ice particles could surround it and then dissapear when the head is back.
And after these transformations a little animation in played out along with the transformed Pokémon's cry.
Of course these are all my visions and I have zero idea if these are possible due to factors I have no clue about. I dunno, Pokémon transformations just feel like a slowly deflating balloon right now and I would like it if they were updated.