Star Guardian Chapter 3
(Ignore the wretchedry that is pajama guardians) I have actually enjoyed these skins as I love cosmic and superhero themes. That said my suggestions for the next SG skins:
Yuumi: as a cat lover more skins for Yuumi can never be bad (just no tech skins though seriously). SG Yuumi would add a new dimension to SG. I know she isnt human. We have so many human skin universes. And as cool as humans are, animals are the coolest. Plus we said SG was ladies only: yet Ezreal is a SG hero now. So dew it.
SG Tristana: just a personal idea. Ik she just got a demon skin but ok.
SG Kayle and Morgana (kayle as the legendary): 2 heroes who fight eachother as much as their enemies
SG Nunu and Willump: a stretch but they seem like explorers so i can imagine them wanting to explore the cosmos and stop evil
SG Diana: vs the void so its obvious
SG Taric: essentially a peace keeper and defender focusing on shielding the innocent from the evil advances of...well evil.
SG Ekko: time bending would be a great asset to the team
If there are other suggestions please say below. Im not suggesting all these be used (definitely use Yuumi tho).
as the legendary, just picture the lines
(suggesting Diana but no Leona, the hell man?) 
is what I'd do
she gets a lot of skins and doesn't have a SG option so she'll get one eventually.
a chance for riot to release a human skin for her, it'd have to be legendary though.
- if someone can have 2 popstar skins, certain champs are going to get a 2nd season makeover.