Based on what I've read here and on the wiki and re-watching the trailer with my anolysts helm on I'm siding with the Akiviri invasion theory. Another voice to the chorus!
The Neverine comes back from his/her expedition to Akivir claiming that the Akivir is now ruled by and swarming with ascended Ka Po' Tun, or dragons. He is dismissed by the Elder Council and the rulers of the provinces (here is where nobody wanted to believe comes in - the only line in the whole trailer that contradicts the clues that lead us toward the Akiviri invasion conclusion). Concerns are towards the bloody civil war in Skyrim spilling discord over into the other lands. The real threat creeps, grows and descends like a flame upon the weak and cowardly people of Tamriel. The Dragon Born, Dovahkiin has some special power allowing him to withstand the Ka Po' Tun and their ascended powers (something related to Akatosh). Nevertheless, all the races of Akivir are soldiers in the invasion force and war rages on.
Skyrim becomes the front line in Tamriel's defense, but help is hard to come by, as the threat is not felt by those far to the South. You hero must assist Dovahkiin in putting the civil war to close and to repel the invaders from a distant land!
Me Sounds like a Mass effect pitch, especially the first bit.
Council member: Ah yes "Dragons" we have dismissed that claim.
a thing or two about this though.
Heros/players in past games tend to disappear, never to be heard from again.
The Eternal Champion, Daggerfalls hero, the Nerevarine, and potentially the Champion of Cyrodiil and the complications with Sheogorath. Many players enjoyed this aspect in a sense and I think it would cause some unsettlement if a past character suddenly comes back and is portrayed in any form, against what the players themselves envisioned.