So what if the necromancers revived Potema fully? obviously it means that the Septim dynasty is revived and she will almost definitively try to claim the throne again. However would The Second War of the Red Diamond begin? Would Titus Mede II managed to defeat Potema? Would the Thalmor benefit in this conflict? And if Potema defeats The Medes, would the Empire prosper under her rule? Would the Empire prosper under future Setpims rule? Would the revived Septims be more popular by the people then the Mede? And would this mean a new era?
Way to many questions to answer succesfuly. But Potema would have right towards the throne and she could claim it, it's likely that a war would start but if the DB killed Titus Mede II then it is also likely that she gains power faster then her opponents.
With an army of the undead and necromancers following her, I doubt she would be really popular in government circles but like in all situations going back to the past will appeal to people so a great deal would follow her willingly, others would do it because of an ancient oath or believes in tradition. The politicians unhappy with the Mede dynasty would probably also see this as a chance for changing things. I don't think Potema would be their first choice, but going for a third party is even harder then going for one of the two, so some powerful people will join her as well. The Thalmor would likely start another war, can't go around destroying Talos worship with a Septim on the throne, and if the Thalmor wouldn't start a war then she would because Potema doesn't seem like the kind of person that would ever allow her subjects to leave her.
As for the Empire, it all depends on who rules. Tiber created an Empire that then spend 400 years slowly breaking down with the occasional good ruler and then a lot of weak ones. If Tamriel is lucky Potema would be a good ruler, but more likely she won't be, and then it will depend on who will follow her. In the near future however there is a large chance of Skyrim and Hammerfall (well definitely Skyrim) falling back in line with the Empire and that would certainly improve the situation. If Hammerfall joins that will improve things as well, but those are short term gains.