To be honest, they need to flesh out a system that allows options for both AP assassins as well as mages. Since both these champions function differently and are currently set to scale off of relatively the same items currently we're just going to see more of the same old same old.
They should probably make each type of ap 'class' desire different stats similar to how its done with the ADs
ad assassins = ArP, CDR and AD
marksman = crit, AS and AD.
AD Bruisers = Generally want AD CDR and HP so they don't get instagibbed and generally have either decent base damage on abilities or methods of damage mitigation.
for our AP varieties we have them all literally wanting the SAME stats to function (AP + CDR with a smidge of MPen brought to you in part by sorc boots and void staff)
By there not being any desirable stat diversity in the pseudo AP classes RIOT pigeonholes itself into making generic stat sticks so they don't absolutely break the 'balance' they have created where things like AP bruisers can't exist. (because i wanna get in there and push buttons and kill things with magical damages)
AP assassins currently have terrible itemisation and revolve around one of two items if not both. These being gunblade and lichbane. Unfortunately because of most mages current inability to burst we have crap like Ahri building gunblade (which mind you is absolutely a disgustingly powerful item).
To fix this they have to either create a new stat or use existing ones to fix it
I would personally love to see real itemisation brought to the different AP classes. Plus how cool would it be to see Champions being able to build some tanky AP items if it compliments their kit.