Someone mentioned this kind of idea a while back:

A healer that does healing in the form of lifesteal. ex: it's basic dps spell is 300 potency and does a 150 potency heal to everyone on the enemies' aggro list or something to that effect. Then just play into that kinda theme while giving it one or two straightforward healing spells.

It's an interesting idea, but I have 0 clue how balancing for that would even start to take form. xD