With the Mage Update changing Zhonya's, I think Seeker's Armguard should be reconsidered.

The item does not fully fit the build anymore. You actually lose 10+25 potential AP by completing Zhonya's (95 -> 70 instead of 95->100), and gain 10% CDR from nowhere.

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Idea 1 (mana regen and is a remaking of the full item):

1. remove the current passive and replace it with UNIQUE Passive: +10% Cooldown Reduction (must be a unique passive since it would be too easy to stack CDR then).
2. lower the AP to either 0 or 10 at most (so you dont lose AP when Zhonya's is completed).
3. give it 50% mana regen since it is neither offensive or team oriented (since it does not fit comfortably into either of mana classes proposed, giving it a small amount of mana regen seems fair).
4. keep armor at 30 (maybe bump to 35 or 40 to compensate the removal of the passive).
5. give it an active similar to where on use (self cast only), removes all stuns, taunts, roots, snares, fears, silences, and slows on the champion (no healing though) with a 90 second cooldown.
6. change Seeker's Armguard's build path to be + 50 gold = 1200 gold (same price).
7. Zhonya's Hourglass' build path will be left the same, but the item's stats will include some small amount of mana regen (passive's on the Seeker's Armguard will be swapped out for the usual Zhonya's Hourglass passive and the 10% CDR passive will become a base stat).

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Idea 2 (flat mana and is more removal/replacement than rework):

1. remove seeker's from the game.
2. change Zhonya's Hourglass build path to be + +750 gold = 3000 gold total (proposed cost).
3. add 300 - 400 mana to Zhonya's Hourglass' stats.