I've been mulling over this idea for a little while, since Sion's default skin is the only one I think even looks relatively good, and I think he deserves a new, maybe even high value skin.
A range of ideas could be implemented or not for this skin, however, the base-line idea is that it's a mechanical rendition. Something akin to The Terminator from the famous movies.
Imagine if you will, a T800 brandishing some sort of stylized axe in place of a firearm.
His R could be signaled with some type of mechanoid sound effect (akin to that we hear in almost every sci-fi movie with mecha like elements in it, from District 9, Transformers, or whatever) with a similar 'boom' effect. Or just be left as is. The animation could be a haze of... "energy/electricity" effect, just for something different, or also be left the same.
He could even be stylized to have a layer of skin which he loses as he fights/takes damage (which doesn't recover until dying and respawning) if you want to get all super referential.
Obviously it couldn't be exactly like the T800 or anything. that would be an obvious breach of copyright, but adding in stylizing with the big jaw element, glowing belly (power source?) and etc.. would work well to separate it.
The differences between a normal skin and legendary or even ultimate skin could be debated, obviously.
I just think the overall theme would suit Sion. "The Undead Juggernaut" or "The Mechanoid Terminator"
Note, I do understand Sion isn't super popular and thus this is very unlikely to get picked up. But considering there are already a fair few mecha themes skins, I thought it worth suggesting since Sion really needs another decent skin other than default. Something that isn't just default with some armour patches added to his model lol