So I want to take a crack at why AP items don't convert as easily to AP bruisers and APC as well as AD Bruisers and ADC.
ADCs use Crit, or some use on-hit effects. Lucian is a bit of an exception as are others, but for the most part if you are "ranged" or an ADC you are using crit.
Their is no AP version of this. If their are AP bruiser items that exist. Lets say one that
Gives AP/CDR and HP with the efffect you get a big Sheild.
Another that gives AP/ARM/MR and gives you cool downs after you use ult like SoS.
And AP/Mana with the effect that all your abilities vamp and have a small AOE (like hydra)
What stops Syndra from being an AP bruiser? What stops Ekko? Cassio? Xerath?
True it may take a few more shots, but with tear and an invincible Xerath, I'm sure you are not having a good time trying to take him down.
It's not so much about what items to make, rather how do we stop X users from abusing it.
I'm sure no one here enjoyed trying to stop Juggermaw, tank Ekko Viktor or Akali.
For me the solution is clear, but I have no idea how much of an over haul it would require. (nor how effective it would be)
Make bruiser specific items melee only, champions like Fizz rely more on their ratios stats rather than base stats. But that's a huge change.
Do I want AP bruiser items. YES
Do I think they can be implemented without huge repercussions or an overhaul? No