Prevent Zed from being able to buy crit, but buff him somewhere else.
Hey. I'm a Zed main since release (almost three years.) You're probably wondering why I'd be suggesting a nerf then. It's simple...
It completely ruins the point of playing Zed and playing against Zed if you can miss your spells and still end up critting your target to death with Death Mark.
Zed is balanced around his shurikens being dodgeable. Zed is balanced around his E and passive being melee range. Being able to crit you to death heavily lowers the skill cap AND skill floor of Zed.
Zed, in order to kill you, needs to fulfill these requirements:
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He must maintain melee range during his ultimate.
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He must hit his shurikens during his ultimate. If he can kill you without them, using just auto-attacks and E (without any crit) then you haven't built any armor.
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He is dependent on you not having items that hardcounter his ultimate.
Notice how all of the above mentions his ult? Because he's that dependent on it. So, assuming we take away his ability to build crit, and understand that his ult is designed with him potentially building crit in mind, then his ult is most definitely the best place to buff if we removed Zed's ability to build crit.
How would we do it? Well, it's easy. Just straight up prevent him from buying items that have crit in them. That's it.
Why is this a good thing?
Because now he actually has to hit his spells on you, and your defensive itemization matters more because he does less damage overall.
Additionally, it opens the gate to buff his ultimate, because his ultimate is balanced with Zed potentially building crit in mind. Thus we can make it go from 20% - 35% - 50% up to 30% - 50% - 70%. (*Keep in mind that exactly how much damage the ult would do doesn't actually matter if you have the proper items, and in addition to this fact, Zed would be far more dependent on his spells than his critical autoattacks to make the damage ramp up.)
TL:DR Zeds who build crit are bad at the champion and need a crutch. This crutch NEEDS to go away in exchange for buffs.