Another MR support item would be nice
I know
is a thing, but I'm talking for tank supports like Braum, Tahm, and Leona.
MR support items:
and maybe
if you have a decent amount of magic damage on your team.
Armor support items:

I am finding it incredibly difficult to itemize to support the team when the enemy does mostly magic damage. Sure, I can build Spirit visage and Adaptive helm, but that doesn't fill the support role at all. Most games as a support tank I will have 250-300 armor and only 100 MR even with merc treads and locket.
40 MR.
55 Armor, 55 MR. Not strictly a support item. But highly supportive in it's utility
since even though it gives more armour than MR, giving a minion immunity to mage damage is pretty effective against primarily magic damage teams.
+
+ 800 = 2400g
? I find that AP Supports have too many MR options, and I cannot say that I have ever felt that tank supports did not have enough MR options. You can also get Merc's (or later in the game swap your Mobi's for Merc's) if needed.