Dovahkiin certainly has the power to create an army of angry Nords and conquer Tamriel, he also knows Dragon Shouts, and he is a Dragon Born - someone fit of the title of Emperor. Also,
Do you think that can possibly happen?
It's also why I think Dragonborn choosing to help Ulfric's rebellion is more canon than helping the Empire - Both Ulfric and Dovahkiin with an army of Nords that know no surrender marching onto Cyrodiil to reclaim it and then straight to Summerset Isle - the heart of the Aldmeri Dominion, or through Elsweyr to weaken the Thalamor. I'm sure that when Hammerfell and High Rock would see the Nords, led by a Dragonborn and another guy who knows how to use the Voice they'll join the cause.
Now for the more lorish stuff --> if that would happen, and the Dovahkiin manages to destroy the Aldmeri dominion and reconquer the whole of Tamriel, would he mantle (into?) Tiber Septim?
Or even way before that *SKYRIM END OF THE GAME SPOILERS*
It's probably just another one of my wrong ideas, but I won't know until you tell me :3
Cheers,
Magico