What about his old kit would even be hard to balance given the current game state? Make his old ult camo, and maybe make q not able to crit if its an issue.. Seriously, his old kit is farless of a gimic/broken.
Not really. Does the current Rengar kit have problems? Sure, but reverting him would just return him to the problems he had before the rework, and stack on new problems with how the game has changed.
But whats stopping you from reverting Fizz too? :]
Rengar needs to be changed, but not back to the old thing he was. Old Rengar was not healthy at all.
Instant burst. Instant vanish. Next to no real counterplay beyond slamming an Ahri E or Morgana Q into his face mid-leap. Stacked his passive too easily on the same target. Essentially, he was as bad as the boards claim Twitch is now.
