I nominate NOCTURNE!!!!!
To be the next cosmic skin. Main reason the cosmic line already has two assassins. next, nocturne only has eye's on his face now compare that with master yi's an kassadins cosmic skin's, also nocturne has not had a new skin since cursed revenant . And nocturne's bio fit's the cosmic theme otherworldly. I will also suggest these 
FOR main colors
Sukishoo1 votesalready has two assassins
Nope. Yi is a Skirmisher.
https://na.leagueoflegends.com/en/news/game-updates/gameplay/taking-another-look-subclasses
WILLOFTHUNDER1 votesnot the point here, And more over negligible difference!!!!
WILLOFTHUNDER1 votesXerath also would be a good cosmic skin
Silvarspark1 votesNah, Xerath would be a better DarkStar skin
WILLOFTHUNDER1 votesmmmmm I would have to agree with ya on that one
MarcoDeff1 votesCosmic Taric would be great.
Hyquiem1 votesCosmic Azir. Smh everything else
WILLOFTHUNDER1 votesNocturne is still my #1 choice, but just for the hell of it. Cosmic pantheon!