I think it would be great to have a 5th tanking class and Blizzard could easily make enhancement a double spec like Feral.
Although Shamans used to use shields a lot more, I could envision them as tanking while dual wielding and using elemental magic to bolster their defenses. Such as earth -> armour, air -> agility, water -> self healing, fire -> aoe threat. They could share many items with feral druids.