As many of us know AP fighter items are pretty much non existent so most characters in this category get balanced around being either 100% mage/assassin or as a tank. Some characters like Gragas, Morde, Diana, Rumble, and now Sylas are adjusted individually with ratio and base damage changes so they can't use items to full effect or the items are simply better on a typical mage. What we need for these characters are items that complement their playstyles and roles instead of number tweaks that simply let them stat check other characters.
Fighters are defined as having relatively good damage and decent durability, wanting to stay in a fight rather than jump in blow up the ADC in 2 sec and pray you can escape. Many AD items give fighters what they need to be relavent preform their job correctly. Items like Black Cleaver, Spear of Shojin, Titanic Hydra, Steraks Gauge. You get my point. All of these items give both the stats needed and also passives that synergize very well with nearly every AD fighters kit. Now we come to AP fighters, and we have....Nashors Tooth kinda? Liandry's Torment? Most items that we would use are often simply better on mages. It's even sadder when on TT we almost have a perfect AP fighter item in Moonflair Spellblade it gives 50 AP, 50 AR, and 50 MR on top of the tenacity passive. Now I said it's ALMOST perfect but it's held back for a few reasons, with how it is currently it would be a must buy for most mages due to its stats and it's just not needed for them. Some changes I can think of to balance this item out would be to make it melee only, this solves most of the mages, now so it isn't just a stat stick I would remove the tenacity and lower the base AP to around 30 and give it a new passive similar to Atma's Reckoning in that you gain AP equal to around 0.5 to 2.5% total hp as you stay in combat. Now I know this isn't a perfect idea but it's around the right ballpark.
Now I know the first thing people will bring up is Fizz and Ekko, the two who have been the most problematic and a main reason to why AP fighter items can't exist. Honestly this is a much simpler fix than many make it out to be either remove/adjust their "get out of jail free cards" (for Fizz personally I'd make his invincibility simply untargetable like Kayn ult and maybe give like 15% damage reduction so he still has some safety and outplay potential but you can actually fight back) or lower their damage enough that they have to make a decision to be assassins and build full AP and penetration or bruisers with HP and resistances. That's about all I got for now if you've read that wall of text please leave some comments below and let me know what you can think of.
Stay metal my friends.