My suggested Master Yi Rework.
Passive: Every other basic attack heals him for his on hit effect damage Q: Stays the Same except remove it's ability damage and instead he "basic attacks up to four targets applying on hit effects" W: Passive: same as current passive "double strike" Active: gives attack speed increase. E: Passive- does on hit magic damage based off his attack speed. Active: (Auto-attack reset) Yi's next basic attack slows the target for "x" percent for "x" seconds but losses E passive while on CD. R: Stays the same but remove attack speed increase and change movement speed into a flat increase.
I feel these things not only fix a lot of issues he currently has but still allows him to be unique as a character. Even if no one agrees with my suggested Master Yi rework, no one can say he isn't in need of one. If anyone has any other suggestions feel free to comment, this is just one of my many thoughts. The biggest problem is trying to make him keep his old feel to him, make him viable, and not make him too strong. Just strong enough to do his job: Late game melee carry. Thanks for reading have a nice day. :)