PBE Rengar W nerf and his future as an assassin
Why do you plan to removed his cc immunity and change it to remove any current cc on him if Rengar's current empowered W doesn't actually remove some until after most cc animations are finished? Ex: Vi's ultimate on Rengar - You can cast your empowered w before she reaches you, but if you cast it when she's on top of you and knocking you up, you aren't able to do anything until after the animation is over even though you casted w when she reached you or knocked you up. I've noticed this affect with his W against a handful of champs (sorry, Vi is the only one to come to mind atm). Also as far as late game, his W will be somewhat irrelevant (as an assassin) because everyone else will just chain cc him instead of doing everything at once.
So are you telling us that you're removing assassin Rengar and promoting bruiser Rengar without saying it directly because you're not entirely sure what to do with him atm? Are you just trying things out for now until you find something that works? Because this nerf would seriously destroy him. He might or can be an assassin early-mid game, but late game it will be almost pointless and Rengar players will have to tank up, becoming a bruiser since the nerf is so inefficient for him late game. I'm not saying I don't see the problem with him always getting away because of his current state, I'm just saying this isn't a good path for him to be on with this kind of nerf. I just don't think this specific nerf is what should be done to adjust (nerf) his empowered W, but I also am not sure what should be done exactly either, he's in a tough spot.
Of course Rengar still has his damage which most people don't like (even though that's just what an assassin is, to be dealing massive damage in a short time to delete someone then use an escape ability to get out), there's still a problem with him being able to get away with this nerf if you can't use anything until you actually have cc on you, but by that time, it's pretty easy to stick to someone like that by just chain ccing them. "Ya, but you can still flash out", an assassin's escape ability shouldn't be useless and make you have to rely on flash constantly, you shouldn't have to wait 4+ minutes just to fight or kill someone again (in a team fight).
[Suggestion]: If you want to make up for him being less of an assassin as time goes on (over time there's been a lot of nerfs, and his rework was supposed to be a big nerf to significantly increase in the time it takes to "delete" someone, but there's still more nerfs to come obviously, if we find a way to become an explosive assassin once again) and stick with these PBE changes that you have, maybe bring back out the ap assassin Rengar and buff his base damage or ap ratios on his W? AP Rengar still works if you play it right, but it's not as good as it used to be (regarding the damage aspect, not so much the heal off ap, even though that was plus).