Malady and you: a comprehensive rework of tanky OnHit items
Core goal: Give tanky OnHit items a unified theme and purpose with potent synergy between them.
So. The Juggernaut patch came and went and Riot didn't touch Wit's End. The Marksmen pass came and went and Riot addressed Wit's End by nerfing it (RIP 42 OnHit damage). So since they seem unwilling to think about this neat item that's sadly neglected, I will. The inspiration here is the new heap of Zeal-based items. A bunch of items that build out of a common component, but all provide you with diverse and interesting choices.
Our new (old) core item will be Malady.
Malady
+ 
Grants IAS, HP and-
UNIQUE passive: Every third consecutive attack against an enemy unit deals (amount) magic damage OnHit.
From here, Malady would branch out into four upgrade paths. Each would focus on helping you survive (and excel against) different threats.
Wit's End
Malady + 
+ 
Grants IAS, HP, MR, OnHit magic damage, and-
UNIQUE passive: Every third consecutive attack against an enemy unit steals (amount) magic resist and deals (amount) magic damage.
Faebane
Malady + + 
Grants IAS, HP, Armor, OnHit magic damage, the Coldsteel passive, and-
UNIQUE passive: Every third consecutive attack against an enemy unit steals (amount) base attack speed and deals (amount) magic damage.
Ionic Spark
Malady + 
+ 
Grants IAS, HP, CDR, and-
UNIQUE passive: Every third consecutive attack against an enemy unit releases a chain lightning that deals (amount) magic damage to up to 5 enemy units. All enemies struck have OnHit effects applied against them once.
Madred's Bloodrazor
Malady + + 
Grants IAS, HP, OnHit magic damage, and-
UNIQUE passive: Every third consecutive attack against an enemy unit deals magic damage equal to (percentage) of your max HP and grants you a (percentage of your max HP) shield for 1.5 seconds.
So as you can see, each item is designed to augment the effectiveness of the others. This would give OnHit damage several viable build-paths to chose from. We've got the tanky physical damage with Bork and Titanic, glassy magic damage with Nashor's and Rageblade, and tanky magic damage with the various Malady upgrade paths.