If I would buff WHM I would give a trait to Cleric
Traited Cleric: Every time you switch into Cleric your next damage spell costs half mana and deals 30% more damage. Every time you switch out of Cleric your next healing spell costs half mana and heals 30% more.
I think it would give them their niche as the best raw healer and best raw damage dealer. (Technically WHM are the ones that do the most damage out of all the healers, their problem is that it's unsustainable)