By having Aegis of the Legion AND Locket of Iron Solari you provide such defense against area damage for your team that it has become the core of my build with Taric (along with Soul Shroud)
Regardless of that i would love to see an upgrade for it. I avoid doing Guardian Angel when i Tank because it makes people want even less to focus me, and that's everything but what i want (i find GA more effective on a carry, actually)
I had this strange idea another day, then i saw this thread, so let me put it here just for sharing.
Bulwark of the Sapphire Vanguard
Aegis of the Legion + Sheen + 515 = 3700
+30 ability power
+25 magic resist
Unique Aura: +10 damage +15 armor +20 magic resist
Unique Passive: On cast, for 10 seconds, your next standard attack deals additional physical damage equal to your combined Armor and Magic Resist. 2 second cooldown.
ps.: i like round numbers