Mastery Level 8 Unlocks 'Master' Champion Skin
You must win 3 consecutive Match-Made Summoners Rift games with S or Higher on an owned Level 7 champion to unlock the Level 8 'Master Skin'.
Once unlocked the 'Master Skin' must be purchased with RP to own and use.
Every Champion would need it's own 'Master Skin'.
The games don't have to be one after the other but would work similarly to promo's in that when you DO use an eligible champion in match-made Summoners Rift games it counts toward Level 8 progress.
Beneficial for RIOT in the sense that almost everyone who unlocks the Level 8 skin will very likely purchase the skin. A lot of players hold off on spending money on things like Skins and many have probably never bought a skin which is how RIOT earns their money. So for first time purchasers it will 'break the ice' so to speak and likely lead to more transactions with the game in the future.
I think it would be a cool dimension to the game and a step ahead in the mastery leveling system,