Riot is thinking about OnHit itemization, so here's a question: What's it for?

As in, why does a champion want to buy OnHit gear instead of AD/Crit or AD/Lethality?

Personally, I don't think trying to balance OnHit gear around pure damage is a very good idea. The reason should be obvious enough. AD/Crit and AD/Lethality both afford better damage against champions and towers while also functioning fully with Lifesteal. So if OnHit effects, which just deal damage to champions and neutral objectives, are going to to compete in the damage department their damage is going to have to be so high to that it's frankly unhealthy for the game (remember Machine Gun Kog?).

Rather than being damage focused, I feel Riot should be looking to and even old Madred's Bloodrazor in terms of what OnHit items should be about. Sources of both defense and offense aiming at mid-game domination. To that end, I feel standardizing OnHit damage from items as Magic Damage would be a good first step. That means you build OnHit items along with Wit's to get the most out of its resist shredding. Bite the bullet on Bork and drop the AD completely in favor of higher %Current magic damage. Give it a new passive that applies Lifesteal to all the OnHit damage you deal. Add two new Recurve items that build from armor and HP so OnHit champs can choose which option will be most beneficial for their lane.

At that point I feel OnHit gear would have a well defined niche. Pick this if you want to be durable while also dealing meaningful damage.