What goes into choosing the right champ for a Hextech skin?

Champs that already have plenty of skins or champs that need more? Champs that are popular or champs that aren't and could use something to draw attention to them? Champs that don't have a skin planned for the year, or is this irrelivant?

I'm just wondering what does into consideration for choosing the right champs for a 10 gemstone Hextech skin?