I had to work really hard to wrap my mind around how a healer could function outside of the regen/shield healing toolset we are used to. The only thing I could imagine is a healer that revolved around debuffs. They would lessen the outgoing damage from a boss or mob and create the illusion of healing by negating the incoming damage itself. It wouldn't be a "shield" for the party so much as poison for enemies. I think it would be neat to see a healing class like that because their skillset wouldn't have any issues with buffs for the party that can overwrite the cohealer.