Shikmari's Shroud Item Concept

So I was sitting in teamspeak talking to a friend about items and what riot may or may not eventually do to AP items/ AD items next season.

Then this Idea came up in my head.

Stat- AP and Armor and unique passive Magic Shroud

60 AP 50Armor (these numbers are really just to kind of give an idea, go ahead and give me some real numbers) Unique Passive: Champion gains a Shield for 30% of the magic damage dealt to an enemy, that decays over 1 seconds. This effect is 33% effective for AoE spells. Max Shield Size grows with Ability Power. Starting with (No Ability Power) 0 - X (X= Ability Power X 4.0). Or Set it to 250Shield - 450Shield (like HexDrinker I guess) (but this would allow champions like Corki to use it as well, sort of. I mainly thought of this item for Strictly the AP champions)

Tell me what you think and how it would effect AP mages.