Wit's end has two issues right now: it has little other items that work well with it, especially for melees. Magic-on-hit is in a poor state ever since S2 for everyone that can't afford to go AP-heavy.
This means it's only usefull on few champs that have enough kit synergie with it.
The other is that it's in a wierd spot where it's to expensive with a poor builtpath(no proper component) to be a really good rush option, yet it's not slot-efficient enough to be great in a fullbuilt.
I don't think it needs aditional other stats, and direct buffs to it's current stats might be dangerous, it'S actually quite costefficient at what it does. Simply buffing it might break it for the (few)current users.
I think it has potential, but needs a bit work.
Imo the best would be to creat a component item, maybe
+
with just the magic on hit and maybe stacking up mr.
Then we could have two different upgrades for it, one that goes the current magic-shred path but more expensive with better stats and maybe one that is more suited for bruisers that want more mixed damage or just the combination of mr and as but don't use the mr-shred that well.
i'd also like to see the on-hit scale with lvl, that way it can beceome decent later on without beeing overbearing then rushed, which else often is an issue with flat on-hit.