I've gone ahead and did a breakdown of the scenes i could see from the Trailer
If you wish, play the trailer and navigate to the timeframes that i mentioned.
I provide my speculations on what they are probably showing.
Overall quite a nice trailer, seems very mature and dark in theme, although doesn't look like
a very big DLC being that vampires and vampire hunters seem to be the main focus so far.
About the Narrator's voice. Did they get someone famous to voice it? Or another Bethesda Voice Actor?
0:05
Shows a fortress across an ocean. Most likely it'll be set in the ocean area north of WInterhold given its snowy look.
0:10
Narrator either says
"You have returned my daughter" or "You have returned. My Daughter"
It appears that that is either the player, or the narrator's daughter.
She also seems to be wearing a cape and has an elder scroll strapped on her back.
0:15
Narrator's eyes look very fiery. Doesn't look like a normal Skyrim Vampire eye texture. Looks very good tho.
0:21
Obviously showing a Vampire Lord transformation as shown in the Gamejam.
0:25
Thats Namira's Altar rite? I dun remember seeing it animated with blood flowing like that.
0:26
New floating animation as a vampire lord
0:29
New feeding animation as a vampire. No more feeding on sleeping people

0:35
Appropriately creepy atmosphere. New sky texture?
0:41
Obviously showing the HQ of the Dawnguard
Anyone recognize if this castle is already in Vanilla Skyrim? Or a new castle?
0:47
Lady firing a Crossbow. She also appear to be wearing a new armor. New Dawnguard Armor?
0:50
The guy also seems to be wearing a new armor.
0:52
I could be mistaken here, but it seems that the Eagle Eye perk now allows you to zoom in much closer. Possible expansion of the current perk trees?
0:54
New gargoyle creature bursting out of a stone
0:55
Falmer getting slashed. But this falmer looks abit reddish then usual.
And looking at the surrounding, i'd say part of Dawnguard's quest will go to
the Snow Elf prince underground ice palace or something? New species of Falmer?
0:56
The Crossbow in First Person Mode
0:57
Player fighting an Ogre? Or a new type of Giant?
The height looks just a little shorter then Skyrim's vanilla Giants.
0:58
New special effects of Bats morphing. Probably a new death animation
1:00
Mounted Combat using an Axe. The horse skin texture looks new.
Horse seems to have a muzzle/harness not seen before in vanilla Skyrim.
1:02
Cool new kill animation as a Vampire Lord. Lift victim up using the "Force" and drain his blood or something.
1:04
New Horse with Blue Fire Streaks. Player also appears to be wearing another new armor.
1:05
Dragon diving into ice. This does not make sense lol and looks abit silly with the poor dragon smashing his head into the ice lol!!
Is it a new dragon behavior? Or a scripted scene? Does this mean that there is underwater combat now?
1:06
Player unfurling an Elder Scroll.
This will be the 2nd time that player unfurls an Elder Scroll in Skyrim.
For players who sold the Elder Scroll to the Orc in Winterhold, what is going to happen?
Is this a new Elder Scroll? The last time we used a scroll, we got teleported back in time.
Will player also be teleported back in time? Perhaps to meet the Snow Elves before they became Falmers?
1:12
Whiterun Dragonsreach appears to be a focal point in this expansion also with that black orb of darkness materializing in the sky.
1:17
I didn't quite catch what the narrator said at the end.
It sounds either :
And the tyranny of the sun "SHALL BE IT" or And the tyranny of the sun "SHELL IT"
Both versions don't make sense lol. Did anyone catch what was being said at the end?
EDIT:
Okay he says "The Tyranny of the Sun Shall End"
Thanks jtuwood for the info.

you know what that means! AVATARS OF MOLAG BALS all round