There is one guy who changed Rengar's kit in the way it has to be.
Like this:
Normal Q - Initial damage lower but boosts next few AAs with bonus damage.
Emp Q - Initial damage lower but boosts next few AAa with bonus damage with bleed damage over time.
Change to his Q makes him more of a sustained rather than split second burst damage dealer.
Normal W - Remove deff stats from his Q and nerf the heal but add some lifesteal to it for x second durration.
Emp W - Nerf the heal, give it lifesteal and make it do damage equal to the enemies missing health
Removes the tanky stats an ASSASSIN do not need at all. Difference between Emp Q and emp W are that Rengar will often use emp W to assassinate targets with lower HP rather than smashing them with Q
Normal E - Should not be changed at all
Emp E - Remove the root completelly, add more range, make it slow the target like normal E but make Rengar able to leap at his target after Emp E.
Change to E leaves opportunuty for counterplaz. Yes, rengar will be able to leap at you but that's because he used his ferocity for the leap instead for the raw damage and also, since you are not rooted you can blink, dash, flash away.
R - Pretty much the same, since it perfectly fits sustained damage dealer Rengar with that MS and fero regeneration after the leap.
Something like this would be cool to see on Rengar.