It's possible but it also depends on how the HP curve continues to go up relative to our healing kit and how fight encounters respond to it. If you're referring to the shield/regen dichotomy, then it's very possible to add in a 4th healer. I think mitigations have to be just as viable as shields for this to happen - which isn't impossible as long as the HP difference continues to rise and shields don't get too strong to enable stacking. After all, White Mage isn't really a regen healer as much anymore but a raw HP healer with the introduction of Afflatus Misery, SCH is mainly a regen/few shields/mitigation healer, and AST is the one with stackable regens/shields but no real mitigation besides Collective Unconscious under Nocturnal Sect.