I think the best start to balancing Zed is changing his passive damage from magic to physical.

That way you can actually counter an AD assassin with AR like you should be able to. He already gets so much APen in most builds and a free 20% AD so he really doesn't need to be dealing 10% of your max hp as magic damage.

This is really just a safe lovetap that makes it more fair when his enemies are rushing armguard just so they aren't 100-0d while he can rush Maw and laugh at mage's damage. If more adjustments are necessary we can go from there.