Rengar...
Before I get started, let me open by saying that Rengar needed to be nerfed. Between his burst potential, level 6 power spike, and lack of feasible counterplay around the state of the Stealth mechanic -- he was overall an unhealthy champ that had an overwhelmingly negative impact on the game for most player, and something HAD to be done about him.
Next -- I'm not a 'Rengod' by any stretch of the imagination (only 229k champion mastery with Rengar), but he Was my most played champion for a reason. He was fun! I enjoyed being able to 1 shot enemy carries. I enjoyed making players feel Hunted. I enjoyed being something that players legitimately Feared if I played my game right and became a big enough threat (and we'll return to this particular point about Rengar later). But unfortunately that's an experience I can no longer enjoy with Rengar...and it's left me completely broken as both a Rengar fan and League of Legends player.
Let me quickly recap the changes Riot made to Rengar in patches 6.2/6.3 -- why I feel they were made, and whether or not I feel they were appropriate changes to make.
Q - Empowered Savagery: DAMAGE SCALING 50% total attack damage ⇒ 30% total attack damage ATTACK DAMAGE BUFF +10% bonus attack damage ⇒ +20% bonus attack damage
This nerf speaks for itself. They pulled damage away from his Q's burst damage (the opening to his 1-shot combo) and gave him more damage for extended combat. Completely reasonable and necessary in my opinion. I have absolutely no issues with it nor do I have any suggestion to improve this portion of his kit compared to where it is currently.
R - Thrill of the Hunt: EXCLAMATION MARK RANGE 1250 ⇒ 1450
Another necessary change in my opinion, especially given how difficult it can be to play around the current state of the Stealth mechanic. Personally, I would implement a global sound that would play when he ults -- similar to Sion / Nocturne R and Corki Package to Further alert players that 'The Hunt Is On'.
E - Bola Strike: Bola Strike now has a cast time (0.25 seconds) when Rengar casts it while mid-air E - Bola Strike: CAST TIME 0.25 seconds ⇒ 0.125 seconds HUNTING PREP Bola Strike now begins slightly behind Rengar when cast while leaping
This is where the champion died... We all know it, we've all seen it, and everyone who's played Rengar since has experienced the frustration that came with this change. But for those of you who haven't -- allow me to explain what happens here. By adding a cast time to the mid-air Bola Strike, you are no longer able to land Bola Strike on enemies that are moving Toward you (something they tried to address with the 6.3 changes 'Bola Strike now begins slightly behind Rengar when cast while leaping' -- but this change mostly impacted the ability to hit stationary targets) due to the fact that by the time the Bola Strike cast is finished, you've landed where your target was when you started to leap -- and if they were running Toward you, they are now Behind you. The behavior of the Bola Strike has changed so drastically, that you're only capable of landing a proper combo on targets that are either Stationary, or moving perpendicular/away from you. I personally took pride in being able to perform proper combos on intended targets in clusters by properly timing the Bola Strike in mid-air. Overall, I feel the changes to the Bola Strike were completely uncalled for (Especially given the other nerfs to his kit) and I feel his E should be reverted to the it's state prior to patch 6.2
WITH THAT BEING SAID, I feel it's only reasonable to address other core issues with Rengar as a champion. Riot has stated their intentions to address the issues around the Stealth mechanic -- but I have a feeling that change is a long ways off. So our only other option is to address his Level 6 power spike and how quickly he can snowball out of control from it. Earlier I mentioned that there should be a global sound played when Rengar ults to alert all players on the map of the impending death that awaits them if they fail to act accordingly. Additionally, I feel that his 'Trophy' passive should be modified to prevent Rengar from becoming too powerful too quickly (as discussed below).
While I admit that I used to use 'Sword of the Occult' while it was in the game, we simply can't compare the power of Rengar's Trophies to the former SotO. With that being said, I don't feel it'd be appropriate to make Rengar a target by allowing his Trophies to be lost like Item Stacks -- Rengar is already targeted by being an assassin that's hated by the majority of the community, and the second he puts himself in a position to be killed, he Will be. Instead, I feel the Trophy bonuses should be adjusted to be given at later parts of the game. For example: 3 TROPHIES => 5 TROPHIES: Unseen Predator Hunter's Domain: 25 bonus movement speed while out of combat or in brush. 6 TROPHIES => 10 TROPHIES: Unseen Predator Unseen Predator's range is increased by 125. 12 TROPHIES => 15 TROPHIES: Thrill of the Hunt Thrill of the Hunt's duration is increased by 5 seconds. 20 TROPHIES => Unchanged: Thrill of the Hunt Thrill of the Hunt's bonus movement speed while stealthed is doubled.
LET'S NOT FORGET THE TANK META!! Now that the Juggernaut items have become so prominent in the meta -- Assassin Rengar is virtually dead, and these changes that were intended to curb the power of Assassin Rengar are completely pointless now.
[Insert needlessly long rant about his Sheen / Ability Cooldown / Bola Strike / etc bugs caused by spaghetti code that should be treated as a critical issue that's detrimental to bringing Rengar back to a playable state]
TL;DR- Changes to Bola Strike were too much, and I feel that they should be reverted and we should find more appropriate means of 'taming' Rengar.
Edit: Let me continue by saying, Yes, I'm aware that Tankgar is a perfectly viable option right now. I've played many games with Rengar since his nerfs and sadly found this to be the most viable option for Rengar at the moment. However, that's simply because the Tank items in the game are far too powerful. In all honesty, I've recently started dipping back into AP Rengar and found that it's an even more viable option than AD Rengar in terms of being able to consistently 1 shot enemies.
I'm just pointing out that Riot's changes were aimed DIRECTLY at Assassin Rengar, the E changes were overkill, and they all came at a very unfortunate time when Assassin Rengar would have died naturally (as he always has in the past) when the Tank meta was just starting to rise again.
(and in games where the enemy takes rengar I make sure to make that point clear to them). Rengar was meant to assassinate your backline. After he does that, anything more he does is extra damage.