I know that the Eternal Yi skin was just released to PBE and is in the works but I feel that you can upgrade the skin to an Ultimate Skin if during his W, all the items with icons that show one sword appeared instead of the base sword of the skin. Like Infinity Edge, Bloodthrister, etc. Just a thought, and I was thinking that because something like that hasn't been done before, something like this would be really cool to have in the game.
I can't really support the idea to "upgrade" Eternal Sword Master Yi to an ultimate skin, it will ship as is.
But the idea you got there is interesting. Having a champion changing parts of his animations based on the items he is using. Especially in combination with the sword items you mentioned, it's really interesting to see models of these weapons to appear.
Kinda has a vibe of "Unlimited Bladeworks" for me. ^^ https://www.youtube.com/watch?v=GkOL55YAz-k
Having an ultimate skin like that would be cool. But I would include these items more into the kit.
For example:
She constantly throws spears, you could change these to magical swords and other weapons. And these changed models would go great with her E.
