There is a reason why you would get sunfire over randuin's in the jungle, and it's based on enemy teamcomp. Randuin's is designed to either supplement initiation with an aoe slow, or cripple aa based champions when you dive them. If cc is not lacking, and the enemy doesn't have an aa dependant carry, or that carry has already been starved, sunfire can be optimal. The extra damage is always nice, particularly in the mid game, and it still aids with clear time. Combined with a lower cost, and it can be a great substitute in the right circumstance.
Typically this fits amummu more than any other jungle, but it can be the right choice in certain niches.
Also, played with the first purchased account I've ever seen. He has 60 cs and is 1/6/3 by 30 minutes as mid despite me camping it, and afks after he fails at stealing my jungle with fizz ults. Turns out he's in diamond with ranked stats that don't match up at all with his recent game stats. Like 200 less farm in equal length games and significantly worse kda on the exact same champ played in ranked. If you're shit, you're shit, regardless of what rank you paid for.
As a former jungle main, I hate FF jungle changes. It made champions I enjoyed playing instantly drop a tier or two.
As a league player, the heal/exhaust changes completely killed the game for me and pushed me out the door to Dota.
I actually like the rengar rework :s roaming rengar was always more fun than lane bully rengar, solely because kitty pounces have to give nightmares to every lane. I think I spend about half my time ganking other lanes as top rengar, the other half just pushing out top with battle meow. Ult comes up by then, so it's off to pay mid or bot a visit with aggressive cuddle ganks. They really should make a kitty cat rengar skin :3
But if league isn't fun anymore, then there's nothing really I can tell you other than best of luck with dota. I still hate valve for not putting techies in that game <.<
Rengar's base damages are more than adequate so long as you aren't maxing q. Scales too poorly to be worth it. Constant ganks and good waveclear should let rengar do a jax-like build of botrk and trinity for better ganks and good burst plus sustained damage, or a tankier randuins with ghostblade for a better lockdown at expense of some sustain and damage. Or the master of troll builds of ie and sotdivine for an ult into sotd, empowered q, q, w, aa for shits and giggles.
Either way, he's still item dependant, but he creates map pressure like if tf and eve had a cat baby. Throw tele on him, and you can gank mid every minute-ish and bot every 3 minutes-ish because of his ult having 120 or so cd at level 1, so like 100 something cd with 5-15% cdr (masteries + items). If his jungle clear was just a little more safe and faster at early levels, I would argue he could be a top tier jungle solely because of his ult and e giving him scary ganks.