Dialogue from the trailer:
"You should have acted.
They’re already here.
The Elder Scrolls told of their return.
Their defeat was merely a delay.
Though the time after Oblivion opened, then the sons of Skyrim would spill their own blood.
But no one wanted to believe; believe they even existed.
And when the truth finally dawns, it dawns in fire.
But, there’s one they fear, in their tongue, is (incomprehensible), Dragon Born."
What could all of this mean? Who could "they" be?
My guess would be Akavir, since they invaded once and were stopped at pale pass, and the Tsaesci were known to have "eaten" all the dragons, which might be why it shows a dragon returning. I do also believe, in lore, that the leader of the Ka po tun succeeded in becoming a dragon.
Dragon born is also Talos, so I think the guiding divinity in the next one might be him. we'll see!