Corki's buffs in 7.12 have brought him over the top
In 7.12, Corki have been buffed significantly, with his base attack speed and attack damage going up and his passive changing his auto attack damage to 20% physical and 80% magical without touching the ratios of his skills.
What this does is that it makes Corki HARDER to itemise against as MR items especially MR/AP items in mid lane is super limited. This poses a problem as Corki building crit items makes each auto attack do magical burst damage on a crit. Coupled with Shiv/RFC, that burst goes higher. So instead of having a burst window every few seconds like a mage, Corki bursts each champion with an AA crit.
This coupled with the fact that the ratios on his skills aren't touched makes Corki a poking GOD, able to put out rapid bursts of magic damage with almost no downtime compared to a standard mid lane poke mage where each burst comes with a few second cooldown. This problem is amplified with the buffs to his base attack speed and base attack damage and the limited amount of MR items, making his damage almost uncounterable.
The reason why ADCs and in turn crit items are balanced is the fact that these damage are countered by armor items and Ninja Tabis which are easily itemised for.
The reason why mages are balanced even with the limited MR item choices is that they can only burst in a window when their skills are up unless you are cassopeia. Cass is considered strong but balanced as she doesn't have an escape.
With Corki? That balance goes out of the window. Now you are facing magic burst damage in rapid succession with him spamming skills and critting your face off with very little itemisation options against him... good luck with that!
TL;DR -Changes to Corki's passive makes his AA and crit damage mostly magical. -Ratios unchanged + the above change = NON STOP MAGIC BURST DAMAGE = POKE GOD -Limited MR items + escape built into kit + the above points = RECIPE FOR OP
EDIT: I realised I may have come off as a whiner without giving an idea for a change. Well, this is what I thought of within 10 minutes.
PASSIVE - HEXTECH MUNITIONS
HEXTECH SHRAPNEL SHELLS: Corki's basic attacks are permanently modified to deal 20% AD physical damage and 80% AD magic damage. ⇒ Each time Corki deals magic damage to an enemy champion, apply a stack of HEXTECH SHRAPNEL. Stacks up to 10 times.
HEXTECH SHRAPNEL: For each stack, 10% Corki's basic attack damage is changed to magic damage, up to 100%. Lasts 2 seconds.
What this does is that it gives "counter play" to Corki. If Corki hits his spells, he does more magic damage to you. This also means that on hit items that does magic damage like Rageblade also stacks his passive, allowing for other item builds.