Skin Trading!
The concept is simple, add a way trading skins between friends.
If there's one change that LoL could use (without getting complaints in the process) it would be the ability to trade skins with other people. I can't be the only one to have multiple skins for heroes that I don't even play anymore. They just sit there waiting for their chance to be used, hoping that maybe I'll go back to that champion and play them again. Having multiple skins for one champ is redundant and having skins for champs you don't play is rather worthless.
Take Sona for instance. I bought her Pentakill skin when the Pentakill album released, and then I got her Silent Night skin out of a Mystery box. Oh, okay, so I have a skin that's only available once a year, that's cool. I can still only use one skin. Then of course Sona gets nerfed to her current state, so now I have BOTH skins sitting there unused.
In other games if you have cosmetics for your favorite champion, but then a new cooler looking set comes out, you don't have to simply sit on the old skin, you can trade it away for a skin for something else. At the same time, you can get something for another champ in exchange.