I have two possible ideas about who this "Dovahkiin" could be:

1. You become the reincarnation of Tiber Septim or Martin Septim
2. Martin or Tiber could return and fight off the Akaviri. (Considering what Ocato said at the end of Oblivion)

It would be cool if Martin could return, as I thought he was the coolest character in Oblivion, although it might not fill in with the Mede dynasty in the Infernal City, unless they were in parallel dimensions or something. (Actually, that's what my roleplaying story is about. I'm also creating a mod that adds the story, in book form, to the Arcane University in Oblivion.

"Dovahkiin" means "Dragonborn", which seems to be a title used to refer to the Septim dynasty (well, emperors in general). I'm thinking the player will be a Nerevar-style reincarnation of Tiber, who as Talos, happens to be an avatar of Lorkhan, or the player will just be a straight-up avatar ala Pelinal. The Dragon thing reeks of Alduin, who is the bad, bad Nordic version of Akatosh, who shows up to end the world and rebuild it every so often.

But the thing is, Shegorath and Jygallag are Daedric parallels of Lorkhan and Akatosh, so that kind of story has already been done, in Shivering Isles.

But so has the invasion plot- what'd be the difference between an Akaviri invasion and the Daedric invasion in Oblivion?

Either way, none of the current Skyrim theories have really wowed me.

I think it'd be cool if you get to rule SOMETHING in this game. (Kind of like in SI)