So, there's always these crazy broken Champions (we all remember Master Yi before his nerf), or those Champions who you never, ever seen played (cough Yorick cough). What Champions have you seen that need a buff or nerf?
frankly, I don't think there's any reason for a champion to not be viable at a certain level. The highest skill cap champions should be the best when played well and the worst when played poorly. The low skill cap champions should be the worst when played well relative to other options, and a weak player should be better off with them than higher skill cap options. thus, I think about 70% of Champions need buffs...
i'm not asking to make every champion an equal pick at professional levels. I just think that the best options at bronze should be very different from the best options at challenger, and that every champion should be considered powerful in at least one division. Right now, I feel like the best options are the best at all levels of play with few exceptions.
To answer your question: every champion that hasn't been picked or banned this far at world's, probably could use a buff - excluding the lowest skill cap champions (e.g. WW, Annie, Ashe, etc...).
Karthus could use a small buff at least. just maybe to movement speed, I can't really think of anything else tho.
There IS one other champ who needs a SERIOUS NERF here, and that's Brand. Every time I try to play against him, I usually get dominated and can't do a friggin thing against him..it's been so bad that I once got super pissed and rage quitted. it's ridiculous how his abilities do so much damage.
Irelia seriously needs to be nerfed, because Irelia.
Also, to further nerf her, every other champion should be buffed, and should have a special aura with the effect "deals 700% increased damage to Irelia". In order to further debuff Irelia, every item should have a stackable passive which says "Irelia takes 300% increased damage from this item. If Irelia herself equips it, every time she attacks an enemy champion, she takes three times as much damage as she inflicted"
Furthermore every item should have an active which insta-gibs Irelia upon being used by or on her.
TL;DR: Better nerf 
