I do agree with the notion that a 4th "WHM but with-" would be more disappointing than no healer at all. Due to how mechanics work I don't see any purely melee healer existing, but one that steps in and out like a RDM would be interesting - Imagine a Geomancer dashing in to shield a party member (you know, like Cover should?), or stomping an area with their hammer, leaving a field of flowers that does healing/damage over time, and then dashing out. Or a Chemist that has Mudra-like mechanic and needs to mix ingredients on the moment and each different combo makes for a different spell or effect.

In short, the dev team has to get creative, AST has enough to make it not look like a [blatant] WHM clone, but that's not going to work a second time.