Yup a mixture of those. And Water fits that very well and is otherwise missing. Thematically earth allows for life (plants) to grow but only should water nourish and wind spread the seeds. Holy (astral/white magic) spells deal with light, life and protection, the same concept as the great bole (tree) and harmonizes with that of the conjurer’s 3 elements.
Fluid Aura is the one, no-doubt-about-it water spell for WHM and is extremely rarely used. I personally only use it in Eureka or if there is a stray caster on a group pull to push it where it belongs. The second reason is why its use (with no element attached and a new name/animation) would make for a better Role Ability (all healers should be able to knock a stray back to the group).
