RDMs basic spells would be really cool to have on a healer.
Maybe add the 3 elements for WHM back and a give him an elemental gauge. Light spells can fill all 3 elements. I though for a while now that Assize would be the perfect skill to deplete such an elemental gauge and add various effects to the party and the enemies regarding on which elements you filled up.
Stone could increase the damage of Assize and give the party a shield. Water increases the healing and the MP recovered. Wind adds a DoT on the enemy... well, it would need some good ideas I think. And the gauge has to cap somewhere so you are actually incentivized to use different spells.