How AP Bruiser items can exist
I've thought about it for it a while, and I realized that AP Bruiser items can't exist without being broken or complete trash, but there's a way around it. The biggest hurdle for AP Bruisers is the fact that they can't **actually **deal sustained damage. Since their autos are relatively pitiful, they must mimic it by stacking CDR to spam abilities. So that's one stat they need; though it implicates spamming, so they also need mana. Being a bruiser, they're likely to be frontlining, so that's also health or defense. They're bruisers, not tanks, so also scale with and need ap. They literally need everything.
A lack of any of these is too weak as seen by our current "AP Bruiser" itemization, but all of it together obviously breaks every mage. What I think is that CDR can substitute some of these stats similarily to how it substituted auto attacks to create sustained damage. If you allow these items to break the CDR cap like Hail of Blades does for attack speed, you can get away with them giving far less AP, which makes them less appealing to mages and hybrids that'd much rather one-shot potential. What I have in mind is an item slapped with a ludicrous amount of mana(1,000+) to burn that's a core item similar to Lich Bane, but, instead of triggering damage, it should increase armor & mr for a duration and/or heal for an amount based on your maximum mana.
Basing it on mana keeps glass cannons from becoming pseudo-tanky like Riven does with Death's Dance, and the benefits of additional armor & mr can only be felt by users that don't die instantly, aka, champions that can survive burst, which means they're probably building hp. The concept forces users to tap into mana and hp.
Followup items should heavily value mana and CDR, offer no more than 60/40/30~ish ap, and have some decent amount of hp which is pretty variable. By doing this, excess mana becomes an important stat rather than useless though implications I see are mages going this route to spam CC and Ryze, Kassadin, and Vlad being their regular selves.
are all technically ap bruiser items, hp and damage o,o
is basically weaker ap 
for example was ap 
the would 2nd/3rd item it like every game
Namely.
Could be easily turned into an AP Bruiser item. And Dont base it off AP to scale autos.
because no health and every AP scaling has to be done with deathcap in mind. Lich as well because has Ap ratio.
which got its range nerfed and have gotten other items nerfed and will continue until you give AP bruisers a unique stat that can't be used by mages.
for a really oppressive all the damage and all the defense combo that will break ap bruisers that areadyy have built in defenses that riot gives them to pad their lack of specific itemization like the shield on 
(which your suggesstion will allow to be almost permanently up and stronger than ever) or the shield on 
broken along with 
and even if you made it melee only would break battle mages like 
Ideal people for your ap bruiser catagory but would have to nerf their Pre getting that item and any build without that item CDR and Damage wise the tie to Mana wont be able to out value the CDR break (if its the same item) and morde would have literal no down time once he got it not that he has much now.