Fair enough - The swap hp for damage would be interesting to see done. In the group I was in it might make battles shorter. OTOH, some of us weren't great at decision making and might have to spend more time deciding how much hp to spend.
It'd certainly make for an interesting puzzle - presumably your defenders would almost always spend a lot of HP because they have so many and healing therefor gives them more.
OTOH, when your AOE types go off, they can hit multiple foes with their spend of limited HP, so maybe they get tempted too