The dungeon from WotLK already shows the roles they would be with red=tank, green=healing, bronze=damage. Even Chromie in HotS is a damage dealer.
Fan ideas are fine, but Blizzard has already established their own rule set as far as what each dragonflight represents and their capabilities.
Blue flights are legitimately the aspect of magic and would fit into a rdps theme, however they embody more arcane than any other school.
Including Black could work, but would be troublesome as right now only Wrathion is considered a good black dragon. Even he is doing questionable things at the times we see him.