How about we make an actual Trinity Force for Tanks?
+
+
=Aegis of the Immortal
Grants a buncha mana, solid HP, good Ar and MR, CDR, and retains its various unique passives. Additionally it grants: Spellblade: After using an ability the next basic attack (OnHit) against an enemy champion heals you for (some percentage of your base attack damage). This ability has a 1.5 second cooldown.
By key-wording the ability in with Spellblade it makes it pretty unattractive to the various Trinity/Tanky Bruiser types. This item would thus serve as a really useful core for a Tank to build the rest of their specialized mitigation around. Because yes, Tanks do build reactively. But it's pretty darn uncommon that the enemy team is going to be totally lacking in one damage type, so having a slot-efficient source of all three defense types makes an important core for Tanks to build around.
Thoughts?
Having a heal on it would make some people pretty OP. You know who I thought would love this item?

and
even tho they don't use the Mana they give so idk
in that build to give Spellblade and a Spellblade effect to heal instead of do damage is awfully OP and doesn't make much sense imo