Wit's end rework?
I love the mechanic of stealing magic resist from your enemy. I think the problem with the item are the stats. Magic resist and on-hit damage make the item very situational, and the only situation where I'd buy it is on Teemo or Azir when the enemy does lots of magic damage. Another problem is that there are other good items for situations like that (example: abyssal scepter).
Some alternative ideas:
Offensive Wit's end (works like the black cleaver 
without stealing MR)
+40 AP
+40% AS
(Unique passive - Basic attacks reduce the target's magic resist by 5% (stacks up to 5 times))
Defensive Wit's end
+60 MR +35% AS (Unique passive - Basic attacks steal 5 magic resist from the target (stacks up to 5 times))
On-hit Wit's end
+40% AS (Unique passive +40 on-hit magic damage) (Unique passive - Basic attacks steal 5 magic resist from the target (stacks up to 5 times))
Caster Wit's end (could work on champions like Ekko 
or Orianna 
)
+60 AP +350 Mana (Unique passive - Basic attacks reduce the target's magic resist by 5 (stacks up to 5 times))
I hope you like my ideas (sry for my bad english),
DarioSiehtFeinde