Honestly I think an overhaul of this level is unlikely. It will get some new spells and some streamlining of its existing ones. I have considered it might become the WAR of healers, having superior dps at the cost of utility thus giving it a value in raid scenarios. However I don't expect anything drasticly different with WHM in Stormblood. They are revamping the combat system, not completely throwing the existing Job identities out and replacing them with something new.