For people who want the video version of Dragonsong with 2.55 Plot spoilers and 3.0 scenes...
For those into the lore, it's pretty clear that this song is a retelling of the story of the start of the Dragonsong War (starting with Shiva and her dragon lover) that's the central element of the Heavensward story, probably as recounted by Hydaelyn. Obviously some of the central elements have a number of similarities to many of the other things happening in the story, as all the many plot threads start coalescing. I'm excited for the expansion for a lot of reasons, but this trailer in particular makes me even more excited to finally continue the story.