I love the idea of the "Star Guardian" skinlines, especially Syndra and Ezreal. However, wouldn't be cool if you could have more Star Guardians, possibly enough to fulfill all the roles of a standard team, cuz Top and Jungle is lacking in the Star Guardian department? I know technically SG Poppy can jungle and SG Lulu can top but what about a Star Guardian Vi, or Star Guardian Evelynn, or maybe even Star Guardian Riven?
Just make into it, and you already have top laner, add in 
and you got a top/jg in one go.
Poppy can top, as it's her primary role, and any of the ADCs can naturally top as well.
As for your suggestions, Vi and Riven just don't fit the lineup, and Evelynn is a definite no, since she's an outright villain. The best bet for a Star Guardian top is Akali and Irelia, and either of them can technically jungle, as can Poppy.
Poppy is a top laner. Jungle you can have Ezreal. I think Vi would be a good candidate for SG, and for top I could see Quinn get a SG skin.
Because no one wants to see that stuff in top lane except people who don’t play it. If a top lane champion got a Star Guardian skin, lots of people would first time top lane with the skin and have a very bad time. It would ultimately be a learning experience, but it would cause much grief all around.
This seasons should be a mix of JG and top laners only. Here's a great squad.
Make this squad happen riot
Poppy is the top laner.
As for Jungler, I'm hoping for SG Kayn
vi nor riven fit the SG theme, evelynn maybe but if zyra don't get it, no way evelynn will since they are evil
Star guardian Shaco would work well.
