Just some skin themes I came up with-
Football Theme- Since it will be hard to make a skin for every football theme there is, just the superbowl teams. Limited edition/Legacy Skins?
Greek/Roman Gods- Some of the Gods that have similar powers/abilities to champpions could be connected. Ex: Pantheon, Ares. Heimerdinger/Rumble, Hephaestus. Ashe, Artemis. Etc.
I think that Riot should come up with Legendary Skins for some of the less played champions to bring them back to popularity. Cho'Gath already has two.
4th of July- I know we have Uncle Ryze already, but I think that Riot can expand on this theme. Maybe for other countries too. Then, you could support a nationality.
I think that of all the ideas I have, this is the most important- Breast Cancer Awareness.
Maybe an all pink character and part of the RP or money spent on the skin would go to the National Breast Cancer Organization. This could help improve popularity for this game and support a Breast Cancer Awareness.
If you think that anything is wrong or have an idea just say so in the comments and I will try to fix the error or put the idea in(with creds to you of course)