Ultimate Skins.
I was having a conversation with my significant other whilst looking through the Marksmen and how many skins they had- and I happened to point out his favorite Marksman, Ezreal. And I said that the Debonair Ezreal splash looked a little bit better than the Pulsefire splash. Since I don't play Ezreal, he told me he wished it wasn't like that in game either, and continued on to say that he wished Riot would put extra effort into skin updates on Ultimate skins to keep them looking like a fresh and new Ultimate skin, instead of letting the skin get outdated and upstaged by newer, cheaper skins. I honestly have to agree with this whole-heartedly, and not just because he is my significant other. I think it would indeed look really amazing to see older Ultimate skins still looking like Ultimate skins. I feel like if Riot were to do this, it would also increase the sales of those Ultimate skins.
TL;DR Update Ultimate skin models so they look like a fresh Ultimate skin and aren't so outdated looking and out shined by newer, cheaper skins!