At this point, I honestly dont' believe SE has an idea how to properly do anything but DPS roles. That is why tanks are officially blue DPS now...

What I would do is, scrap the whole concept of the healer role and replace it with the support role. The support role could be anything from a melee, to physical ranged or caster and would be a DPS/heal hybrid. DNC would perfectly fit that role already (with a few tweaks).

This has two big pros:
1. You can give the support role a full DPS rotation,which either buffs the group or debuffs enemies and give them 2-3 heals.
2. You can make tanks have an identity besides "blue DPS"!
Make them be responsible for their own survivability, f.e. by giving them active mitigation they have to keep track of while tanking and give them the tools to keep themselves alive.
The group dmgreduce CD also already fits into that concept.

This is what they could do:
SCH
Debuffs enemies.
Gives out heals/shields through the fairy.

AST
The SCH/WHM hybrid.
Rework the card system to either hand out buffs or debuff enemies (diurnal/nocturnal).
Make each card unique again!
Draws could be achieved by completing their DPS rotation.

WHM
Buffs the group with white magic.

All of this has to be refined of course, but I think this concept is something that could work out really good, with how battles are desingned in this game.