If you want to improve wits end, give it some HP
Pure resist items are really bad right now and the variable value of Wits usefulness from game to game doesn't really help. Adding HP for more universal defense and slightly adjusting the effects it gives would go far for its overall viability
I actually think adding a Phage to it would be really useful since a lot of Wits potential users have poor synergy with the other two upgrades or just can't really afford it on their budget (Volibear, Warwick) or generally doesn't particularly care for the effects (eg Shyvana doesn't really care for Trinity but greatly enjoys Phage)
I think the teased Zephyr rework was a solid direction for it and could be good for Wits to follow up into; gaining increasing MR at a much higher value, with a higher flat magic damage to compensate for no MR shredding. Wits would make a solid high tier 3k+ gold item with a setup like that.
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make the champions insanely snowbally and capable of 1v5ing which is just unhealthy and stupid for the game. Adding an updated version of Wit's End where enemies get Armor+Magic resistance per auto will make it impossible to kill the already hard to kill Master Yi and Jax.