Ok from all the crazy bits n pieces of dlc info I've been able to put together a possible story.

Your character gets introduced by rumors of an island not too far from the northern coast of Skyrim. A few weeks earlier an expedtion had been sent to this island and has not been heard from since. So after being sent to the island you chance upon what looks like a small, ruined village. Everybody looks like normal villagers with a blacksmith, a couple of decent traders, and high-figures. Someone tells you that this is the expedition and that they have been unable to send any messages back to Solitude due to "sea monsters". Finding this odd, you know that you havent run into any sea monsters on your way to the island. Later on in the day, someone suggests that you stay at the Black Fang Inn claiming that it offers some of the most luxurious and sincere delights that anyone has to offer. While sleeping you hear strange noises in your room. Waking up, you see a couple of villagers entering into your room. But not just normal villagers...vampires!! Armed with (new weapons and armor) they make an attempt to kill you. After easily dispatching them, you hear lots of noises outside. Going outside, you see that every villager in the town is a vampire. Not just that but they are also fighting werewolves! Both sides run to engage you and you easily dispatch them one after the other. You heart begins to race with the excitement of the dlc! There are white and black werewolves, you see cool new vampire animations, and you get to test out your first crossbow!!

Eventually getting overwhelmed you are instantly teleported out of the battle and into a small room in another building. There you meet...the Snow elf prince himself! (not knowing this yet of course). He offers to take you to a hidden refuge of his kind. You eventually get introduced to the dawnguard, a vampire/werewolf organization that hunts vampires and werewolves. And then, the dlc main quest begins....

That is my theory on what the dlc might be about. What did you guys think? I'd like to see what story you can conjure up so comment below!

This might make sense... Except both Vampires and werewolves prey on humans. It'd make no sense for a bunch of them to be sitting on an isolated, hidden island.

soo.. How did you come up with this? Did they release any info.. I know this is just a thought but any evidence to support this?

Its all in the dawnguard threads. Thread 30# page 3 if you want to see for yourself.Below is some of the information from that page

DLC in patch 1.6. Key features:

• Animations of crossbows now moved to folder DLC01.
  
• Mounted combat with crossbows.
  
• At least one type of crossbows will be unique with unique animation - Dwarven.
  
• Child Crying.
  
• New dialogue gestures:
  Defensive Hand;
  Hand on Chin;
  Welcome Hand.
  
• Coffin Puzzle.
  
• Reading of the Elder Scroll.
  
• Animation from the trailer of Dawnguard.
  
• Scraping tree bark.
  
• Transformation in a Vampire Lord!
  
• Vampire Lord's kill moves.
  
• "Serana holds Vyrthur". Of course Serana and Vyrthur are in-game NPC's (or something like that), but I have not found any mentions of these names in TES Lore.
  
• Snow Elf Prince ascension burning.
  
• Pour water in a way shrine.
  
• Scooping water in a way shrine.

If you know Russian language, you can read Russian translation of the list above on my site - http://www.elderscrolls.net/news/index.php/weblog/dawnguard-16/

Also, here is the full list of the DLC entries:


Spoiler

Animations\DLC01\Chair_IdleVampireEat.hkx

Animations\DLC01\Chair_SnowElfPrinceBigSpellIntro.HKX
Animations\DLC01\Chair_SnowElfPrinceBigSpellLoop.HKX
Animations\DLC01\Chair_SnowElfPrinceBigSpellRelease.HKX
Animations\DLC01\Chair_SnowElfPrinceDialogueA.HKX
Animations\DLC01\Chair_SnowElfPrinceDialogueB.HKX
Animations\DLC01\Chair_SnowElfPrinceDialogueC.HKX
Animations\DLC01\Chair_SnowElfPrinceDialogueD.HKX
Animations\DLC01\Chair_SnowElfPrinceDialogueE.HKX
Animations\DLC01\Chair_SnowElfPrinceFireBall.HKX
Animations\DLC01\Chair_SnowElfPrinceSitIdle.HKX

Animations\DLC01\Child_CryingEnd.hkx
Animations\DLC01\Child_CryingLoop.hkx
Animations\DLC01\Child_CryingStart.hkx

Animations\DLC01\CrossBow_Aim.hkx
Animations\DLC01\CrossBow_AimDwarven.hkx
Animations\DLC01\CrossBow_BlockBash.HKX
Animations\DLC01\CrossBow_Equip.hkx
Animations\DLC01\CrossBow_IdleDrawn.hkx
Animations\DLC01\CrossBow_IdleDrawnDwarven.hkx
Animations\DLC01\CrossBow_IdleHeld.hkx
Animations\DLC01\CrossBow_Release.hkx
Animations\DLC01\CrossBow_ReleaseDwarven.hkx
Animations\DLC01\CrossBow_Reload.hkx
Animations\DLC01\CrossBow_ReloadDwarven.hkx
Animations\DLC01\CrossBow_RunBackward.hkx
Animations\DLC01\CrossBow_RunBackwardLeft.hkx
Animations\DLC01\CrossBow_RunBackwardRight.hkx
Animations\DLC01\CrossBow_RunForward.HKX
Animations\DLC01\CrossBow_RunForwardLeft.HKX
Animations\DLC01\CrossBow_RunForwardRight.HKX
Animations\DLC01\CrossBow_RunLeft.HKX
Animations\DLC01\CrossBow_RunRight.HKX
Animations\DLC01\CrossBow_RunStrafeLeft.HKX
Animations\DLC01\CrossBow_RunStrafeRight.HKX
Animations\DLC01\Crossbow_ShoutOffset.hkx
Animations\DLC01\CrossBow_TurnLeft180.HKX
Animations\DLC01\CrossBow_TurnLeft60.HKX
Animations\DLC01\CrossBow_TurnRight180.HKX
Animations\DLC01\CrossBow_TurnRight60.HKX
Animations\DLC01\CrossBow_WalkBackward.hkx
Animations\DLC01\CrossBow_WalkBackwardLeft.hkx
Animations\DLC01\CrossBow_WalkBackwardRight.hkx
Animations\DLC01\CrossBow_WalkForward.hkx
Animations\DLC01\CrossBow_WalkForwardLeft.hkx
Animations\DLC01\CrossBow_WalkForwardRight.hkx
Animations\DLC01\CrossBow_WalkLeft.hkx
Animations\DLC01\CrossBow_WalkRight.hkx
Animations\DLC01\CrossbowSprintForward.hkx

This is what I get from everything I've read...

I think the Dawngaurd will be strictly a vampire hunting society.
Werewolves have the Silver Hand on their backs already.

The player will be able to join them or become a vampire lord (there may be a vampire faction of some sort) and oppose them.

Either way both sides are seeking something connected to the Falmer's past, so expect caves.

The Dawngaurd's weapons of choice will be crossbow for medium ranged attacks and some new 1h melee weapon of a holy variety, likely a dagger... maybe even a wooden stake, with a unique kill move... plunging it into a vampires heart!

30+ threads is a little beyond TL;DR for me, so thanks for compiling the info and providing a possible theory from them. It sounds interesting; though werewolves and vampires in Skyrim don't do much for me since there's no compelling reason to play either on any of my characters. I would love to see that lycanthropy skill-tree actually make it into the game. My werewolf never shifts, because when he does, he's weak as hell. And doesn't vampirism just turn you into an attack-on-sight villain everywhere you go? Nevertheless, I'm anxious to play me some Dawnguard.

This is probably the best concept of it i have heard. The Dawnguard a group much like the Vigilants of Stendar, but the Dawnguards are actually worshipers of Meridia.

I believe the snow elf prince will be the one that offers you to become an apprentice vampire of his, and if you say no, a fight breaks out, if you agree to, he has you hunting and destroying all the Dawnguards.

Twilight Cliche.

anyway, this island must be... ATMORA! I KNEW IT!

If you haven't fed in 4 days, people can see what you really are, if they can see you.
Stage 4 is also when you are most powerful and able to move through populated areas with relative ease when you've been doing it for a while (Embrace of Shadows, the shadow stone etc).
Guilds also don't mind that you're an undead bloodsvcker (DB obviously sees it as an asset), so it's quite possible to do a lot of business as usual at stage 4.

Vampires are seriously underrated in this game, werewolves get all the attention.
Maybe Dawnguard will change that.

I'm thinking vampire lord might be a new stage beyond 4 triggered by something unique to the DLC,


Once you transform into a lord feeding will no longer reset the cycle, it progresses it.
Lord unlocks a perk tree that marks progression from this point on.

For a perk tree that doesn't rely on normal points from regular leveling I think maybe lords can feed any time on anybody and every time they do the tree fills (with blood, muahahaha) and after a few necks you hit the various perks along the branches and choose to unlock A or B or whatever.

If you don't feed the bar goes down, relocking perks as it drops (but this is a slow process and can be halted and/or reversed by simply feeding to keep "topped up").
If it drops below the equivalent of zero the vamp perk tree vanishes, you revert to stage 4 and feeding will once again reset the entire cycle as normal.

The flip side is, lords suffer much worse penalties, fire damage in sunlight ftw.


All that said, this is idle speculation at best... maybe the DLC will have nothing to do with vampires at all.

Heh, half right.

Bloodmoon anyone?