I digress! A trailer disection that will yield results some of you may not have thought of
You should have acted ... they're already here... The Elder Scrolls told of their return
To me, in context with what was being said, it is not dragons. Following the Lore, Dragons were very friendly creatures to the nords, which makes me think it is either a new threat (new to us, old to Skyrim itself) OR It is the Mythic Dawn (more on that later)
Their defeat was merely delay to the time after Oblivion opened
This is what makes me think it is the Mythic Dawn. They were the ones behind the gates opening all over the place, and the gates were closed by you, and sealed by Martin. So I highly doubt there will be any more Oblivion gates. But agaun, this could be something new.
When the sons of skyrim would spill their own blood, but no one wanted to believe ... believe they even existed
This obviously refers to a civil war of some kind, people supporting different factions and causing a divide. Again, referring to this new threat, which somewhat disproves the Mythic Dawn idea. It can't be Dragons (Im not saying they can't be in teh game, I'm saying they can't be teh bad guys)/
When the truth finally dawned ... It dawns in fire ... but there is one they fear ... Dragon born.
Now! With all of that put together, i put to you this; The Akiviri have set sail and have landed.
Now with the reference to the dragons I think this is it. Not a literal dragon, but the people from the Land of the Dragons (Akiviri means Dragon Land). An assault, a envoy, a something to Tamrial. I do believe that there may be one dragon (they can't NOT have a real dragon now can they ) But I think we shall see some involvement of the Akiviri.
Just my 2 cents.