I have the feeling either way the empire is going to fall and we're being set up for a Cuhlecain and Tiber Septim style reformation of the empire. Whether it's Ulfric or Tullius leading is probably something we'll never know.
It'll be Tiber Septim himself who reunites the Empire and squashes the Thalmor. Due to one of the
very few leaks i've seen of Dawnguard, i believe the Dovahkiin learn's the "Slen Tiid Vo" shout that Alduin uses to ressurect long dead Dragons. He then uses the shout on Tiber Septim's remains (Tiber Septim was a Dragonborn, therefore having the soul of a Dragon), and brings him back. Tiber Septim then proceeds to reunite Tamriel, then once worship of him is restored (and worship fuels his power), he uses his powers of CHIM to sink the Summerset Isles beneath the waves, forever ending the threat of the Aldmeri Dominion.