Lower Armor on Zhonyas
It already gives AP champions an ability other champions are unlikely (I say unlikely because you technically can build it on anyone) to build, stasis. It doesn't need to give as many resistances, especially stacking resistances as it does.
Okay, the argument that there are AD+MR items may be made, however, AP is a BURST damage. It can 100-0 you while (most) AD champions need to sustain the damage they deal to kill you.
Move the Armor Zhonyas has over to another AP item, the stasis and resistance creates an unfair advantage, especially to AP tanks.
Thanks.
. I understand your concern with the matter, but you also need to realize there are certain champions (specifically mages in the midlane) that purchase Zhonya's as their core item during a match specifically because of it's active and armor it grants.