One issue with Holy Warriors in DF is that the other fighter-types, generally, do better against undead or demons than they do. A Knight or Swashbuckler with Weapon Master exceeds them in skill and damage, even against those the who the Higher Purpose applies. Barbarians do so much raw damage that they exceed the Holy Warriors as well.
One solution is to allow Holy Warriors to have Weapon Master. Sure that'll work.
But another idea I had is to make Higher Purpose do per-die damage bonuses. So a Holy Warrior with Higher Purpose 3 vs. undead would have +3 to hit, +3 to defend, and +3 per die to damage. A ST 13 Holy Warrior with a Fine Broadsword would do 2d+7 to undead.
You could allow this to stack with Weapon Master if you are a glutton for such things. Or just allow this instead.
I haven't tried this but I like how it plays out in my head, so far.