It would be nice to have more Armor based utility items for Supports.
As a support, if you want to itemize against a heavy ap composition, there are a lot of MR options available that also provide utility like

But really, the only utility based armor item is
(which is a nice item in certain circumstances!)
Things like
are great if you're thresh or leona, but don't make much sense on Janna or Nami.
with the popularity of Ad assassin midlaners and tanky bruisers top and jungle, it would be nice to have a few more option for buffing your team against heavy ad comps.
Of course one could always pick the outrageous Gem Knight. 
What are your thoughts?
Edit* Just to clarify, I wasn't saying frozen heart isn't a good item for supports at all, I think in certain situations it's really great.
I'm also not saying that Randuins, Thornmail, and Sunfire are terrible on Caster supports, only that they aren't generally going to be part of your standard build, without sacrificing part of your utility or ap scaling.
Edit 2 Forgot to mention
as a utility based armor item!
should have different enchantments like jungle items and boots. One enchantment for MR/Armor, one for AP/AD, one for movement speed, one for health regen, whatever you need.
+
+
+ (any)

into a late game
is a fair build path and it works especially well against AD assassins. The utility comes in on ratios on heals and shields (there's a good reason people buy
on Janna) and hourglassing as a supprot can often be very powerful eg. a flash ult from Sona followed by the hourglass so she doesn't die.