Just an Idea I'm throwing out there for the next Ultimate skin
So for Spirit Guard Udyr, he gets almost a whole new skin for each stance. Pulsefire Ezreal changes his augmentation as you level up. DJ Sona has three different looks you can toggle through and you can play electronic music ingame. Elementalist Lux is essentially 5 skins and 5 more chromas wrapped up in one.
But the one ultimate skin we don't have is one that changes look based on the champion's current killing spree.
Clearly, this would work best on a carry/assassin of some sort, since supports and - unless you play them well - tanks hardly go on high killing sprees.
It would probably thematically fit best on an evolution-based champion such as
or
.
I have no real idea what it would look like, just the general idea. And I'm flinging it out there for you creative minds to work with!