I have watched it a few times but I cannot say I am impressed. It looks like you won't have much choice in the DLC. Vampires want to block out the sun, and you have to stop them. I would like there to be the ability to join them and block it but if you think about it, that would ruin the whole game and be an automatic endgame.
I'm pretty sure there's bits of the Dovahkiin turning into a vampire lord there, and the narration also very obviously suggests a choice ("Is it to be one of us ... or are you with them?"), so we clearly get two options.
I don't see how blocking out the sun is an "automatic endgame," either. It'll just be like regular Skyrim, but 200% more dark.

Besides which, I think Bethesda learned their lesson about forcing an endgame on sandboxes with Fallout 3...so much so that they "fixed" it with DLC. Add in the fact that there's clearly more DLC to come, and it seems pretty silly to put an endgame in the first DLC you release.