Unfortunately I don't think Riot really has a good grasp on what they want OnHit itemization to do.
Phantom Hit suggests Riot wants to try to push it toward having more late game scaling. Problem is, if you're an auto-attacker then AD/Crit/IAS is going to be superior damage output all the time forever barring weird kit effects (farewell, machine gun Kog W).
Right now, OnHit is basically 'here's a thing that gives auto-attackers a mid game power spike I guess?' which isn't terribly compelling.
Personally, I would prefer to see Riot try to give OnHit a unified identity. Items like Wit's are, I believe, a good foundation. Wit's would be pretty decent if it had other items it could pair with. If there were more tanky magic damage OnHit items for Wit's to augment, then suddenly it wouldn't be restricted to just champs who already have magic damage OnHits to toss around.
I would say make a 'new' component item. Let's call it Malady. Malady builds out of Recurve, Dagger, and a combine cost. It gives 40% attack speed and 30 magic damage OnHit. Malady could then be combined with a defensive item (Negatron for Wit's, QSS for Dervish Blade, Warden's Mail for an attack speed stealing item, Giant's Belt for Ionic Spark, ect). So what does magic damage OnHit become? A gaggle of IAS/defense items. The sort of stuff that a bruiser would want to build.
One goal here is to make all the items augment each other somehow. Wit's shreds resist. Faebane (Warden's Mail item) steals base attack speed OnHit. Ionic Spark splashes OnHit effects to secondary targets. Maybe have a new version of Madred's that grants magic vamp in the same fashion that Death's Dance gives physical vamp. Basically, everything feeds into everything else. In terms of raw damage, Crit/AD would still win hands down. But OnHit would allow beefy front-liners to eat some focus and still dole out damage.
Barring a total re-haul, I think I would be comfortable giving Wit's ramping OnHit damage rather than a MR shred effect just so it would be more attractive to a wider range of champions. I would still miss the flash shred, though. Wish Koggy still had his flat shred on Q, honestly. Eating 70 MR from people was hilarious.