I have a hard time imagining a necromancer without undead minions to summon or a variety of nasty plagues/debuffs etc to use. So I don't think a traditional necromancer would work well as a healer. I also don't generally think of a necromancer as healing anything without a cost. Maybe 'transfering' life harming one foe to heal another. But I really doubt SE is going step that far out of the box for a fourth healer to have a healer who heals via dpsing.

I think necromancer would work best as a casting dps if they decide to add a fourth one.