Idea for Gragas skin
Here's an idea for a "free" skin.
A Gragas "moneybags" skin, where he looks kind of like Scrooge McDuck, and carries bags of gold coins around. He throws a bag of money instead of a barrel of beer, and it explodes in a shower of coins with a coin tinkling "ching" sound.
This skin would be given only to players who have purchased all champions and all skins. If at any point you own all champions and all skins, you permanently unlock the "moneybags" skin, as a reward for being a big-spender and supporting the game.