It's not a very safe assumption to make regarding opening up other classes to Dracthyr. It might take some work, especially with spellcasting animations, so I'd imagine that the first classes they'd open them up to would be spellcasters -- Mages, Priests, and Warlocks, since they lend themselves towards the combat style of the Dracthyr better. Everyone else would come later, since they'd have to come up with new rigging for melee/tanking (and hunters, in particular given bows/guns).
But to say that it will never happen is asking to get mocked heavily and openly since you don't work for Blizzard, nor can you see into the future.