So Vastaya basically League's Yokai then? Also about retroactively making champs Vastaya
That is, fantastic magical creatures of all sorts of shapes and sizes. Shapeshifting's even a thing. Makes even more sense when you take Ionia's Asian influence into account.
Their shape is defined by magic.
In the same way our shape is defined by water.
Does this mean they could be just about anything? I think this is a poor analogy (sorry WAAARGHbobo) but it does leave a ton of room for creativity. If magic is to them what water is to us then that puts them in a whole new category of life. The possibilities are endless. For example, a magical fox who longs to be human. If vastaya are so broad so as to be simply "creatures of magic" we can retroactively make champions like Ahri into vastaya without really changing their original lore/stories beyond adding a label to them. Their origins/backstories wouldn't have to change, we'd just get something extra to call them, and I suppose it could have future lore implications as well.
I think those of us worried (myself included) are worried about characters we love like Ahri suddenly becoming something else, something they weren't. But if vastaya are what's described above, then nothing's actually changing about them. They could have always been a 'vastaya', we just know that's what they are now. Rengar can still be a lion-man-thing, Ahri could still be a magical 'fox' turned almost-human, the otter tribe can be their otter tribe, etc.