Rengar's problematic crazy snowballing.
I think one of the problem with Rengar's kit is how easy it is to murder people with your ultimate with little to no counterplay. Find someone not with his group? Ult and get a kill.
Rengar being able to leap without using bushes makes his ultimate too strong. The "eye" warning doesn't help much besides letting you know you're about to die.
One way they could fix this is cutting the leap's range by half if no bush were used during his ultimate BUT also changing his proxy warning to a global warning. When Rengar uses his ultimate, everyone's screen turn red'ish. You know he's prowling but not where. If it's too big of a nerf, make it so he can normal range leap on targets at 50% hp or lower during his ultimate.
This wouldn't hurt his overall kit much but would reduce some of the cheese his ultimate gives him. It would also allow Rengar to put global pressure when ulting. Currently on live, if the proxy warning goes away you know Rengar is backing off. With this change, you would not know if rengar is backing or if he's still lurking around.