I think there are a lot of things Blizzard could do to add depth. Dual classing for example is one of them. Even having a free-form skill system where you build your character through choice and create an unnamed class that falls into your own choice of archetypes.
The issue is that nothing seems to persist between expansions. If the did, each class would have multiple sub classes associated with it. I would personally like to see the class system go somewhere closer to Pathfinder / D&D. Where players can make good and poor builds, butt there are so many options with synergies that it is hard to pin point a specific meta.