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Perhaps, you could add some more legendary skins to the store, though only make them available for IP.
I mean, you'd have to play a lot of games to reach the amount of IP needed, as they'd cost between 10,000 IP to 60,000 IP, making it hard to grab them all.
Perhaps if they did get them, they'd also get a Summoner Icon as well?
This way, when people see they have the skin, they know they're dedicated to the game.