The nerf Lee Sin mains don't want you to see!
Lee has very few clear weaknesses, which makes it difficult to find a nerf that accentuates any weaknesses.
The only real weakness I can think of is that it is important for Lee to land his q. Counterplay against Lee depends upon dodging his q. So how about we increase the penalty if it misses?
Give his q a substantially increased cooldown if it does not hit a target. If his q does not hit a target, put safeguard/iron will on an 8 second cooldown.
With this nerf, Lee sin will lose a huge amount of damage if he misses his q (only his e and maybe r will be available, which also gimps his passive). In addition, he loses his escape. So now if lee misses a q, you can really punish him, at the very least forcing him to ult.
This nerf would not fundamentally change lee's kit, but it gives counterplay and forces lee sin mains to be more skillful.