I get that everyones real big on Rengar with his 48% winrate making him somehow completely unplayable and gutted, but corki is kinda actually unplayable right now. The last change is going to affect him more over time but the earlier nerfs left him kind of dead. I get his play rate is really low but he is my favorite champion. i agreed that when he had a 53% winrate mid he was kind of a problem, but that rocket nerf dropped him 5% and kicked him out of mid lane and he really doesn't have anywhere to go because it's even worse bot lane.
I think making his big one do more damage and reducing some of his mana cost would be good changes that wouldn't break him. For one, his Valkyrie takes 100 mana which is the same as most ultimates. On top of that, it also has a massive cooldown. I think making the mana cost lower at early levels would be fair and maybe shaving off 2 seconds or so on the cooldown. As for the big one, it should just get a simple damage increase. When I saw the first nerf, I thought that the big one was going to do more and the smaller ones would do less but they just all do less.
Also this 80% magic damage thing is really bad. I don't think we'll see problems with it for a while but eventually it's going to fuck him really hard. For one, building mr against him before was already immensely effective. But now it's all you have to build and he just doesn't do any damage. When we hit a tank meta and tanks have 2 or even 3 mr items depending then corki won't do any damage regardless of whether or not he buys void staff. With how bad corki is and how the change didn't help or hurt him that bad currently, the change should really just be reverted. If you're going to do 80%, it would make more sense to do 80% physical damage.