I don't inherently dislike your idea. Never played Priest in WoW, nor played long enough to get a feel for the meta. What you're describing sounds like it would, itself, be a class that has specs, and after SCH and SMN, SE has sworn off of that. As far as "it could queue as either role," SE avoided that possibility with RDM and BLU, I think they're avoiding that idea as much as they can.

If Elementalist were 100% healer it would still largely be able to fulfill your fantasy of a healer that does damage. It just can't be a healer that can take the place of a DPS, or vice versa, because then you end up with it as an almost mandated job in the meta due to how DPS focused this game is.