So they're focusing on balance for now, and waiting until 6.0 to add a fourth healer. it is what it is, but how will we balance these jobs? We have a 'pure healer' and 'shield healer' and a combination of the two, and that's the main problem.
My suggestion:
Split the healing paradigms up into healing modes and mitigation modes:
Regens: Set up a heal over time, a regular amount at a standard rate, more reliable, less control
Preheals: Set up a delayed heal that triggers a certain amount of healing after time/circumstance met, less reliable, more control
Resistance: Sustained mitigation, mitigating a percentage of all incoming damage, more reliable, less control
Shields: Set up a damage buffer, mitigating a set finite amount of damage that is expended, less reliable, more control
Give each healer their own combination of the two:
WHM - Regens/Resistance
SCH - Preheals/Shields
AST - Regens/Shields
??? - Preheals/Resistance
This way, each healer has it’s ‘niche’, each healer has complete function for light-party content, and any combination of healers can be coordinated similar to how twin-AST currently is.
One healer choses to focus on regen/preheals, the other focuses on mitigation. This is because regens/preheals would clash(can’t time your preheal if HP is ticking up) and resistance/shields would clash (resistance is wasted if damage is completely absorbed) but healing/mitigation wouldn’t.
I think this sort of set up is very possible.
If you take Protect away from the others and make it WHM exclusive again, put it on a cooldown and buff up the mitigation, that’s your main ‘resistance’. Add in Shell as a single target version and you’re done. Ideally, WHM would also need a complete Lily rework, and these could tie into the functions of Shell and Protect, and ideally provide better mobility, with instacast Cures or something.
AST is pretty much there already, and SCH would simply need a little more focus on this Preheal concept, maybe tweak Selene to provide it, add in something else similar to Excogitation or Emergency Tactics.
They would still keep their gimmicks, WHM is the GCD healer, AST has cards, SCH has the pet.
This would mean our fourth healer would be a Prehealer with Resistance buffs and another ‘gimmick’.