It'd be hilarious to release a sports skin for Karthus

I can imagine only a few sports would work,

Golf master Karthus would summon a golf ball for his Q and it'd be wacked by a spectral golf club as it explodes

Tennis champion Karthus would summon a tennis net and wack tennis balls over it for his autos

Cheerleader karthus would have firework effects for everything and blush a lot (ooh sempai)

Pro Karthus would be a basketball skin where he's dribbling a basket ball the whole game, when he ults he spins the ball on his finger. (special interaction with dunkmaster darius)