Free skin token for champion mastery 7
You should get a skin at champion mastery 7. Either a token to redeem one in the store, or just one that's automatically given to you. If you put in the time and effort to get M7 on a champion, you should have a skin for that champion.
Doesn't even have to be a good one. Just whatever the cheapest one is, or the most unpopular. Or maybe create a suite of skins for champions that you can only unlock by getting them to M7
Master skins. Master Taric, Master Amumu. Just some re-color skin. Anything is fine, really.
Maybe just a golden re-color.