Ares! Two Chains! Well... if you miss the third one,

Though your core build is good, it seems you are more interested in the situational late game items. That being said, I would like to see greater reasoning and discussion on the various choices.

Also to note, you posted this as a support tank, but some of those combinations seem to be leaning a bit closer to a damage tank. Why not make two different tabs for a tank and hybrid tank based on the replacement items? Or simply name the one build "Situational Awareness" instead? Regardless, this is a little to broad and you might need to narrow it down to a few items for either a tank or damage build.

Here are some of my thoughts on the items, just to clarify if I understand your reasoning:
Polynomicon + Gem of Isolation means you can basic hit the target in between Shackles and Searing Flesh when close enough (damage tank)
Breastplate of Valor if many enemies grab Aegis Amulet or Beads of Purification (should assume the would though and get early game to max No Escape effectiveness)
Pythagorems Piece if you have a lot of squishy magic casters on your team who need help staying alive, but still want damage
Obsidian Shard if enemy has a lot of magic defense and your ahead enough to go damage (again straying from support)
Bancroft's Talon or Rod of Tahuti if you're finding yourself in a lot of one - on - one engagements (though I would not bother with Bancroft's Talon without first getting Rod of Tahuti as Rod is a little more powerful to begin with and 12% lifesteal isn't that great)

Why do you like Midgardian's Mail and Stone of Gaia on Ares? Stone of Gaia would help with heavy magic protection, but then why get it as your very last item? Same with Midgardian's Mail; the passive doesn't seem to great once you reach late game and there's much more team fights than just basic attack beat downs. I do however like your choice of Magi's Blessing over Bulwark of Hope. I'm assuming because Bulwark of Hope only helps when your already nearly dead, and only resets every 60 seconds, while Magi's Blessing is active more often and negates the damage entirely.