Fixing Wit's End in the Preason
Wit's End isn't actually bad as a completed item in it's current iteration; it's niche on certain champions and that is a very good thing. In fact, if you run the numbers according to the Wiki, certain champions pay 2500 gold to get 4870.25 gold worth of stats. Of course, this is just a stat stick unlike Zhonya's or Banshee's and doesn't provide the best defense like Banshees and SV do. Still, it's quite strong yet you rarely see it built. Why?
Wit's End has a terrible build path. A Recurve Bow, a Dagger, and a Null Magic Mantle leaves you with very poor offense in most cases as you have no AD to make use of the AS with (as most people who build AD avoid this item because it's magic damage does not scale with crit or abilities). Further, Banshees and SV (which Wit's competes with) have build paths that give more MR and health to boot! So this item is really terrible when built, but shines on niche champions when it actually is. Fun fact, according to the wiki, 2370.25 of Wit's End gold worth is in its passive! And because none of the items it builds out of have such a passive, it usually is left out when considering build paths.
How can this be fixed? Get rid of the Recurve Bow in the recipe. Recurve Bow IMO is rarely seen as it builds into three incredibly niche items: Runaan's Hurricane, Wit's End, and Sword of the Divine. I do believe Riot is also removing Recurve in the new Season. What a perfect time to change the recipe! What if there was a new "Advanced" item that gave some stats of the final Wit's End? What if this item could be applied to another item's build path as well?
Voodoo Dagger: Dagger + Longsword + 390 gold = 1200 gold (OR) Dagger + Dagger + 300 gold = 1200 gold 40% AS UNIQUE: Basic attacks deal 15 bonus magic damage on hit. Builds into: Wit's End, Nashor's Tooth.
In the Wit's End recipe, this replaces the Recurve Bow and the recipe cost now drops by 300 gold (to 450 gold) so that Wit's End remains a 2500 gold purchase. For Nashor's Tooth this replaces Stinger and Nashor's recipe cost is increased by 50 gold (to 900 gold) so that Nashor's Tooth remains a 2920 gold purchase.
EDIT: This could just be a rework of Stinger which is one of those very rare items into my above suggestion (keeps the name "Stinger" though). Would only need to update the extremely rarely bought Zehpyr if that was the case! (Seriously, when is the last time you have ever seen anyone buy Zephyr???)
Tl;dr- Replace the Recurve Bow (which is being taken out of the game in the preseason) in the Wit's End recipe with a new item that gives some AS and a weaker version of the final Wit's End item. Could possibly be used in Nashor's Tooth as well; doesn't have to.
and make the build
= 
has: Excellent completed item, but a power plateau while putting it together. If you've got enough money in your pocket to straight up buy it, then it's great. But if you try to buy it piece by piece, you're only hurting yourself.
It's beautiful!
(on-hit changes to physical damage),
, the new Devourer enchantment and possibly
if they every decide to remove
.