Problem with Rengar is that when he's strong (for the majority of the player-base) he's complete bullshit. This comes from burst + stealth. Either he dives in, one-shots the carry and proceeds with fighting and getting away or he dives in and dies without contributing much to his team. If Rengar is to be a strong assassin they should remove the stealth.
edit: As a guideline, this is probably what the Rengar rework back in patch 4.5 (Season 4) should have entailed:
- Removal of Stealth
- Reduced dependency on ferocity. Ferocity should enhance his ability to perform a specific task but not be necessary to doing so (example: His burst is low without ferocity Q, but he can't stick well without ferocity E. Ferocity W has a good use case in healing but is not necessary).
- Remove him being a tank assassin when ahead, make his durability/ability to survive more skill-based. His would usually mean either a spell-shield / auto-attack block or a very short duration (~1.5 seconds) shield that can be of high value (like Riven).
- Removal of BTN as an item and introduced as a trinket/buff attached to the champion (they already did this and was one of the few things successful of the rework).
- E as a skillshot (again, they did this and it was another successful part of his rework).
Basically all the Rengar rework did was introduce some mechanics as counters to his bullshit (the ! on his Ultimate and his E being a skillshot) and make the Bonetooth necklace available throughout the game (and as bonuses or enhancements to skills instead of stat-stick) which slightly reduced his feast/famine gameplay.