IMO, id prefer they add more specs to the game instead of classes.
Think about it, they dont have to worry about past content not meshing with new classes, like order halls, the new specs can still use all the armor sets from past tiers of their class, they dont have to create a ton more art like they would for a brand new class, and it could add more roles to dps exclusive classes.
My biggest wish for next xpack is more roles, starting with…
Shaman- Earthwarder, tank spec
Paladin- Righteousness, Ranged DPS spec.
Rogue- Bard, Healing spec
This way, the two classic classes will have access to all 4 roles (4 because ranged and melee dps), and people will finally get bards which to be frank would be hard pressed getting 2 or 3 specs as its own class.