For WHM, I think an easy buff would be adding a "Holy Bar" which does the following.

Holy Bar increase when you cure targets, however it decreases when you use magic spells.

With the following bar you can do the following:
Holy gains special effect based on how high you built up the bar.

50%=Dmg potency +50
75%=Cast no MP and has a no resist Stun
100%=Adds Regen to allies and a DoT to all mobs hit by it.

By adding something simple like this, you create flexibility that WHM can play with to be more engaging. And the "Holy Bar" doesn't even need to be just for Holy, but the above would encourage focusing on healing while offering perks to DPS and utility by using their signature Magic Spell Holy.