What we have learned from Worlds so far
First off,
needs a serious nerf. She is 100% pick/ban so far and an 83.3% win rate. This is pretty similar to what happened at Worlds last year. If nerfing her will make her to weak or unviable, then she needs a major rework. She is simply too strong for her role. Gut the cat so we can see some actual diversity in the jungle. Gragas wasn't as strong and you nerfed him.
Secondly,
needs serious nerfs. She is 95.8% pick/ban (it would be 100% if all mids knew how to play her) and has a 71.4% win rate. Her kit has so much utility that she doesn't need the obscene damage.
Third, 
. If you nerf Nid, you have to nerf Elise and Lee since Nid is the only one really beating them. Elise and Lee have the same problems as Nid as well. This would open the jungle back up to a lot of viable picks.
Lastly, the win rates on
are really high and they probably need some slight nerfing.
was 100% pick/ban last year at Worlds and got completely destroyed by nerfs shortly after, remaining in the dumpster for months until the balance team gave him some pity buffs.
was in the same pick/ban boat and got handed consistent nerfs for basically the entirety of season 6.
and 
needs to be addressed more than some of the other champions mentioned. He is frustrating to play against as a lane bully and being manaless, low cooldowns, and scales pretty well after two items. The only issue he has is sustain in lane, but he can easily kite top laners away if the player is good enough.
and
the most, but that is because I main
. Fiora just is a better duelist even if I kill her early and I can't slow Olaf so he dictates the lane pretty much and can pick if he wants to dive or not.
clg freaking wiped the floor with them...