I've been playing Arena of Evolution a lot lately, and all skins they have give 1% up to 10% bonus Silver Coins for each one you have in your collection.
And I thought to myself: "What a cool idea! Definitely makes collecting skins MUCH more enjoyable. Rito should implement a similar mechanic, but with Exp instead. Would make skin purchases much more tempting."
The numbers obviously should not be too crazy. Like 1% Bonus Exp for each Common Skin, up to 5% Bonus Exp for each Legendary and 10% Bonus Exp for each Ultimate skin. I really can't see any cons to this rather than very few salty people that like to Grind for Exp without paying money. But seriously, people that like Exp Grind are weird to begin with.
+Us, the consumers, would only get Faster Leveling since many many players already have a lot of skins in their collection.