I once saw a custom skin for Varus, that did something very interesting. Whenever your passive would activate, Varus changed color. I think in normal mode he was purple, when he killed a minion he turned red and when he killed a champion he turned yellow.
The creator said that is possible, because of the way Varus' skin/passive are programmed.
So I asked myself, couldn't this be used for a legendary (or maybe even ultimate) Skin?
Something like a Demon/Bloodmoon-Varus that would look normal, then turn a bit demon-like when killing minions and when he kills a champion he goes full-demon-mode (like High Noon Lucian during his Ult).
Yeah I know we already have an "evolving" skin with Elementalist Lux. But this would be something different because you don't just pick a form in your base, but it responds to your gameplay.
Ideas for other themes beside demon/bloodmoon:
Arcade/Battle Boss (this isn't even my final form)