you know, i just joined the stormcloaks, and on my way to whiterun to deliver the axe i got to thinking.
i feel like the dragonborn is in the perfect position to really mantle tiber septim if he sides with the stormcloaks. it's just like tiber and cuhlecain, the dragonborn could take skyrim for the stormcloaks, then backstab ulfric and blame it on the thalmor, and then take the ruby throne by a mix of force and the bend will shout on the elder council.
the dragonborn is stronger than tiber at his peak(without numidium of course(CHIM is iffy, nerevarine was able to defeat vivec with CHIM)) and knows at least four shouts that tiber didn't(soul tearing, durnehviir, dragon aspect(actually more powerful than it appears due to the metaphysics of bringing out the soul of a former dragonborn), and bend will).
if bethesda has the balls to create a canonical dragonborn, or even just leave them ambiguous but set some form of canon better than "disappeared on solstheim teehee", then we could be on the verge of a new empire.