If a champion has 48-52% WR then IMo they are fine. If they have a 51+% winrate combined with an above 15% Play Rate, then they should begin to be looked at. Ban Rate also factors into those considerations, as a champ with an abnormally high ban rate, play rate with a decent win rate might need to have balance considerations taken since a champ with 100% P/BR should not have above a 50% winrate.
Also, the meta figures into these considerations, especially since Riot is not looking into actively fixing or shifting it atm. The current meta calls for high burst and mobility, with less emphasis put onto tankiness or DPS. Scaling champs, crit users in particular, are going to have a lower WR.
You can knock 3 champs right off at the start. 
are never going to become viable without large kit changes. They provide too much emphasis to the user and none to their opponents. WHEN zoe finally receives substantial nerfs that her kit deserves, she will be in the same category.
has a lower WR because games dont go as long as he wants them too, plus he lost a large chunk of raw damage in the IE changes. His barrels already ignored % of armor, he doesnt want true damage, he want MOAR DAKKA
will never be strong in solo queue because if hes OK in SQ he is a god in team play.
All the tanks that you have listed up there have poor WR atm because no one plays a tank when the meta is all about level 2/3 burst with electrocute+ignite adding ~178 damage at level 3.
Which leaves us with Karma, who was overnerfed because she has both ranged burst and shields (nerfed for poke, then nerfed during the shield support nerf patch.) and Gnar who just isnt as strong a sidelane deulist as Fiora or camille, the reason he is picked in team play is because he can provide both side lane pressure AND front line engage, which makes him incredibly tough to balance as a champion.
Saying that pros are the reason that a champ gets nerfed or overnerfed is misleading. They can just take a champs skill ceiling as high as it possibly can and expose flaws that most players aren't able to ever see.
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