Just my two cents,
I believe the dragons referred to in the trailer are actually those of Akavir (meaning Dragon Land, see Mysterious Akavir), the landmass East of Tamriel which is inhabited by many beast races including the 'Ka Po' Tun', the 'Tiger-Dragon Empire'. Skipping a small amount of lore, the leader of these people called Tosh Raka had physically transformed into a dragon with the colouring of a tiger. Even before the trailer starts the Akavir are hinted to with the loading symbol of an Akaviri inspired dragon image, the same found in The Imperial City (on the gates, statues and Imperial Watch belts)
Mentioned in the trailer by the voiceover is "their return" and that "their defeat was merely a delay", the unnamed party mentioned are most probably the Akavir. This is because the Akaviri had invaded Tamriel before and were swiftly defeated at Pale Pass. This could mean that the Akaviri are once more returning to Tamriel and that the defeat was only a delay to another invasion of Tamriel. Also mentioned in the trailer is that the "sons of Skyrim would spill their own blood" this could be hinting at a civil war or conflict happening within Skyrim at the time of the invasion.
Another interesting piece of lore that I found was that the Nerevarine had travelled to Akavir and never been heard of again. What I'm about to say is probably the most incorrect thing but it's just my little insignificant thought. The "Dragon-Born" mentioned may be the Nerevarine or a descendant of them, this I highly doubt (and you should too) but nonetheless something to think about.
Totally speculation at this point, just what I have picked up from lore and the trailer.