the main problem about wits-end is that a part of its power-budged is in reducing the enemies MR. that may sound good, but the only attackspeed-reliant (or at least benefitted) AP champion in the game (aside from offbuilds like AP Warwick or so) is diana and she would always prefer nashors over wits end.
this means, the reduction to magic resist this item gives you now only benefits its own damage, which isnt very high.
like, the only champion i ever buy it on is nocturne jungle. i go devourer, botrk and wits end (sometimes) which then adds up to a crazy amount of mixed on-hit damage and it also benefits my devourer too, but thats like "meh" and getting wits end over ANY other damage item (youmus, BT, Hydra, BC) is VERY situational and im not sure if worth.
i mean, we have an upgraded brutalizer, an upgraded sunfire, an upgraded wits end and a downgraded lichbane in the jungle, and the role that favored wits end the most was the jungle allready, so since you can get devourer, it renders wits end obsolete for many champions.
a really good thing for wits end would be to clarify its identity more. we have many strong attackspeed items in the game, but wits end is too hybrid to benefit anyone. if you are an ability-user, you dont need the attackspeed and if you are an auto-attacker without magic damage, you dont need the MR reduction.
what i would like to see, IF anything at all, then it would be to amplify wits ends role as a MR-reducer, for example, it should steal your champion level in magic resist per hit, up to 3 times for 5 seconds (meaning on lvl 18 you can steal 54 MR). also, its magic damage is basically lower than that of nashors tooth, and nashors scales with something. i think wits end should also scale, and it should be like 35 base + 40%% of your bonus mr. now that sounds alot, but magic resistance is expensive and you would never really want to have more than 200 anyways, and that means you allready are somewhat full build and high level and invested quite alot into it.
so, its final stats would be:
50% attackspeed
unique passive: (35+40%bonusMR) magic damage on hit
unique passive: basic attacks steal (champion-level*1) magic resistance from the target. this effect stacks up to 3 times and lasts for 5 seconds.
as you can see, i removed the flat magic resistance but increased its potential on-hit damage as well as its stealing effect to make up for it. the item now is more an offensive item for auto attacking magic damage dealers with a defensive portion added to it instead of being an offense/defense hybrid.