I think the elementalist theme shouldn't be for Lux, but for Ryze
Well, Lux is a great magician, able to learn many magics, in that regard she is quite good for this skin, but we have already known her as Light mage, the most logic step would be (at least in my opinion) a light spectrum skin, were she dominates the light, enough so she can divide it in every wavelenght of the spectrum, even the non visible ones.
I do think the elementalist skin should have been for Ryze, even if in him the skin would have less flashy particles. Lorewise he is an ancient mage, specialized in rune magic, runes has always been related with elemental magic, but it seems this experienced rune mage can't mix elements and rune magic.
I want to make clear, even when i don't like Lux, i think the skin is great, and even more on her, but i can't let that wasted potential go unnoticed.