Ghaul is more likely to hold a rank of Legatus. He might put the former emperor to be in exile as a result from a coup d'état. But that does not automaticly grants him to become the Imperator. As the legitimacy of the title need to have total control of the military and approved by the senates. That would give him more reason for him to lead the Red legions to earth and bring back the Traveller as a trophy back to the empire, which if this mission succeed it would help prove his righteous and be notorious to all the citizens of the Cabal empire. Ghaul is leading the Red legion which I assume to be lesser than the Imperial legion, also to go out to battle is cutting tie with the empire, so that would make him have no more political position within the empire. If the whole Cabal empire went all out to battle earth as a last resort, we would have to see a giant fleet of the Emperor along with the Imperial Praetorian Guard, Senatus, Legatus, Tribunus, Praedectus and Primus. That woud be something with 40 hours game playand, awesome story plot and great dialog and at least 2 hours of epic cut scene. Which I am not convince that Bungie could deliver that at the moment. Ghaul just another Primus wanna be Legatus.