Frozen Heart would be more appealing if it provided health.
If we traded some of Frozen Heart's stats for health to put it in line with other tank's itemization, it could easily become a niche counterpart to Randuin's Omen and Thornmail for tanks who want to protect their team and not just themselves.
The item that is currently the closest to Frozen Heart functionally is Abyssal Mask, which provides health, mana, magic resist, and a debuff aura that strengthens allies by debuffing nearby enemies. If we shape Frozen Heart up to be an armor counterpart to Abyssal Mask, we get something that looks like this:
Frozen Heart
+350 health
+50 armor
+300 mana
+10% CDR
UNIQUE Aura: Reduces attack speed of nearby enemies.
UNIQUE Passive - Eternity: Damage taken from champions is gained as mana. Spending mana restores health.
+ 
I think it would be cool to have an armor counterpart to Abyssal Mask with the Eternity passive to turn tanks durability into mana sustain. It would still be a niche item, but more universally appealing, and unless Riot changes its passive so that it no longer has to compete with other Warden's Mail upgrades, this is about the best we can hope for.
Thoughts?
and
, I'd much rather it give more armor. 100 or 110. The unique passive is fine as-is since it combines with the other
and
together are really gold inefficient for the redundant passives, but are often too necessary for lifesteal and crits of marksmen. To combat this, it would be nice to see each of the two offer something extra from the passive, so the best of both apply when stacked.




