AP Bruiser/Battlemage Item idea

So, the problem with AP bruisers and battlemages right now in general is that they don’t have an item like BC that gives them stats they need and encourages them to stay in longer fights. However, an exact AP version of BC with stacking mr shred would just be picked up by every mage and abused. So I propose a solution:

New item:

Builds out of haunting guise + kindlegem, provides modified passive:

UNIQUE - Madness: while in combat with an enemy champion within X units (idk units, but think like 2-3 teemos or less), decrease their magic resist by 4% every second for the next 5 seconds (capping at 20%). Leaving aura range will cause stacks to decay each second by the same amount.

The numbers may be off here, but the idea is that AP bruisers and battlemages are encouraged to be in longer fights (which is already their role). At the same time, while mages CAN pick up this item, it means that if they want the passive they have to be able to be close enough to enemy champions long enough without dying. Essentially, if mages want the benefit of being a battlemage, they have to be able to survive as a battlemage. I think this sort of trial by fire is the best solution, because other options I’ve seen involve making it a melee only item, which would be a drag for champs like and who are battlemages but not melee