The reason AP itemization is so much harder to both differentiate and balance it's that AP champions have 1 less stat to care about compared to ADs.
Coming up with a new stat though isn't that easy also because it should feel like only a certain group of APs should be looking for it over the others.
The only one I could come up with works more rather as a new items' passive than a stat but it might be a good start:
_**Empower**_: Casting an ability that consumes mana empowers other abilities granting them a bonus that scales with the numeric value of _empower_ you have.
This bonus can either be one between (or we can have mages choose a kind of empower based on the first empower item purchase) more raw damage, increased range, faster tick rate for dots, a high cance not to trigger the skill CD (only apliable to basic skills though) but causing the next cast to deal less damage/less CC duration.
Any more brainstorming is welcome.
**Edit**: I’m not saying that AP items are worse and should be buffed, I’m saying that it might be healthier for the game to have another AP focused stat in order to make AP items more easy to balance and differentiate mages’ builds.