Currently lower scope skins are being designed in groups. We brainstorm a theme that could work on multiple champs (Dark/Light Knights, Dinosaurs, House of Cards, Academy) and then choose a bunch of champions that meet some criteria (work well with the theme, will look good without effects/animation support, haven't had a skin for a while). We do a rough spread of concepts of those champions and what they could look like then, based on those sketches, we choose the strongest skins from among them to go into production.
Higher scope skins like Star Guardian Lux require a lot more work from a bigger number of people. A group of 5 star guardians would require a huge amount of work and mean the team couldn't work on any other skins like Legendaries or Ultimates for a long time. So we can do it, but it's it's a big investment in a single theme so we'd have to be very confident that it's a theme a lot of players are going to enjoy.