Riot should be designing champions that fit the following criteria: fun to play, not incredibly difficult to balance (looking at you Ryze), and most importantly valid counterplay by champion design.
-fun to play
Champions should, at some level, appeal to a certain audience. There are champions of different roles, lores, and playstyles that define them. For example, players who are drawn towards assassins may find champions like Katarina and Talon fun to play. All champions in league should be both unique and fun in there own special way.
-Not incredibly difficult to balance
Ryze has been through an insane number of reworks simply because of his base champion design. His numbers constantly need tweaking because he is a feast-or-famine champion, meaning that either he is incredibly overpowered or crushingly underpowered.
Champions should be strong **if played correctly.** I find that by herself, Morgana's W is extemely powerful as a waveclearing and laning tool because of its immense damage. I think her W needs a nerf... but that's not what I'm here to talk about. Let's get into the core of this thread:
-There should be valid counterplay by champion design
This means that there should not be a champion where the only way to defeat it is by outplaying them, since this creates an immense skill requirement.
TWO CHAMPIONS WHO DO NOT HAVE COUNTERPLAY: Riven and Rengar
Riven has insane damage, CC, and mobility, the "tri-force" that makes a good Riven an unstoppable beast. Add in a Death's Dance for some lifesteal/healing and boom; Riven is a broken champion.
There is no counterplay to Riven other than to hope that she messes up her comboes.
Rengar is a completely bogus champion simply because he can use 4 abilities in less than a second and burst you in under a second. Once he uses his ultimate and you (as a squishy) have the indicator above your character, you can count your blessings and say hello to a 1950's black-and-white movie of watching your team get decimated. The only valid counterplay I can think of in relation to Rengar are items: GA and Zhonya's.
TL;DR: Riven and Rengar are poorly designed champions that need to be revisited/reworked to provide a funner experience for both players playing rengar, and playing against rengar.