The split has been a bizarre design decision from the start. I have no idea why they did it. The only possible explanation I can come up with, is 'they did it so that the WHM players could say 'I will focus on just healing''. It closes so many design options off from the Pure Healers, like Stoneskin is gone, Noct Sect is gone, etc. And in a game so focused on mitigation and 'have enough damage reduction to avoid getting oneshot by this', this basically results in Pure Healers feeling somewhat less useful compared to the Barrier Healers. If SCH/SGE shields stacked (Galvanize/E.Prog), I'd expect that every hardcore Ultimate/week 1 Savage group (ie with players who use 'the strongest option' rather than 'their favourite') would run SCH/SGE. because Mitigation is just so much more impactful in raids (unless 7.2 changes something)
It's a fine balance to walk between 'the classes feel homogenized' and 'the combat feels homogenized' but I'd argue that allowing all healers some form of GCD AOE Barrier (eg bring back Stoneskin, and readd Noct Aspected Helios in some form to AST), while a small 'homogenization' in the short term, allows for a lot of diversity in combat design. Say a raidwide applies a bleed, but if you apply a barrier, the bleed is negated (the raidwide still does damage though). Do you A: Regen through the bleed, B: Apply a Barrier, negate the bleed, and heal up the raidwide damage? The choice wouldn't just be 'which healer am I playing', but also 'do I have the resources to tackle this in the way I hope to?' For example, a SGE has Panhaima and Holos as OGCD options to apply a Barrier. But if those aren't up, do you still go for the barrier option via E.Prognosis, even though it costs a GCD? Or do you use Physis and Pneuma to heal through the bleed instead?
Instead of a hard split, I don't know why SE didn't go with a sliding scale. For example, they want WHM to be bad at Barrier/Mitigation. Sure, that's fine, just make it be the healer that's the furthest along the scale towards the Pure side. But that doesn't mean it should have zero options for AOE shielding (no, Divine Caress doesn't count), because then you can't make certain design elements (eg, the example above with the raidwide bleed).
If you have a sliding scale like:
Pure ------------------------- Barrier
WHM > > AST > > SGE > > SCH
Then it's super easy to add a new healer into the mix. You just adjust its design to balance the amount of Pure Healing (ie burst healing actions like Star, Cure3, Pneuma, etc) versus how much access to Barriers and Mitigation it has (eg Expedient, Soil, Panhaima, Holos, etc). So, my suggestion would be, have a healer that has two 'stances', ala AST sects - a heavy focus on HOTs in one stance, and a heavy focus on (rather than a standard Barrier like Succor) applying a buff to allies that staggers part of the damage taken out into a DOT. The HOT would counteract the DOT, stabilising the HP level of the party. And thus, said healer would slot into the scale at the very middle, between AST and SGE. If said healer were partied with an AST or WHM, it'd be able to focus more on the mitigative side of its kit, and if it were with SGE/SCH, it'd focus more on the HOT side of the kit.
But let's say SE decides that actually, AST should be the midpoint, because they're bringing back Sects. Then, the new healer could have more of its kit designed to be Pure in nature and less Barrier related, and then it'd slot in between WHM and AST.
Divine Caress, SunSign and Seraphism make it feel like they're silently acknowledging that the split doesn't really work very well, at least