Kayns ult has so much more creative potential
He point and clicks to "infect" someone then breaks out and deals damage. Thats it?
Something more thematic would be stealing AD, or perhaps weakening the target so they take more damage from Kayns attacks for the next 5 seconds, something like that.
As of now its so ... boring, and lacks any counterplay. There could be a kind of button-mashing minigame where he can extend the duration and therefore the buffs for himself, and the victim can have counterplay to reduce it.