Its nice idea to have healer 3 rankings, like you said ... healer, tank, dps.

Anyway matching system also need rework. Currently its working based on avarage ranking. Eg you have one gold in team, enemy got one gold in team, you have one unraked enemy will have unranked.
But it can create situation when gold healer is matched against golden dps while rest of team is unraked or bronze(silver, etc). That will create bad situation.
It should be made way that gold healers are matched against gold healer dps against dps and tanks against tanks.