I would love a 4th spec based on a combination of skills/talents from current PVE/PVP abilities from all specs of a given class. Kind of like a jack of all trades master of none of your own class that represents the class and not be limited to a single spec. Essentially a hero spec. Some of the PvP abilities are so cool I think they should be allowed in instances as part of this hero spec.
Edit:
OK, I think the jack of all trades master of none wording is not exactly how I should have presented this idea. Maybe just a hero class version of a given class but in the form of a spec unlockable at max level via an epic quest line.
Edit #2: If, like some are saying, balance becomes a showstopper for such an idea to come to fruition, then make it an illusion of a spec, meaning copy another spec as-is, keeping the backend the same, and change spell animations and ability wording (pretty sure they already do that....)