When I got to Morrowind I said, "Hmm..." I'm taking it with a grain of salt. I won't get my hopes up.

What i think "Dawnguard" actually is, is a newly formed, specialized sect of the Vigilants of Stendarr that is particularly trained to fight nocturnal monsters, such as Vampires and Lycanthropes. Lamae Beolfag awakens from a centuries long slumber, and whips the Vampires of Skyrim into a frenzy. The Volkihar and Sanguinare launch increasingly bolder and more brutal attacks on each other and mortal settlements in an attempt to earn Lamae's favor. The Vigilants of Stendarr form a specialized group called the "Dawnguard" in an attempt to stem the slaughter of innocents, and fight back against the Vampires, eventually taking the fight to Lamae herself if you join the VoS.

If you're already a Vampire by the time you activate the plotline, you get an invitation from your clan to join in on the slaughter and glory. Vampirism will have 2 seperate strains in the expansion, "Sanguinare Vampiris" is contracted from the Sanguinare Vampires that live in/near cities, and "Volkihar Vampiris" can be contracted from Volkihar Vampires that live way out in the wild. The Sanguinare clan is a Stealth based clan, while the Volkihar are a Warrior based clan, with their abilities reflecting such. Both have some magic enhancing abilities. The Sanguinare have their Illusion based magic enhanced, while the Volkihar have their frost based magic/shouts enhanced. Both will also be able to feed in combat by casting "Vampiric Drain" on a near dead NPC, initiating a feeding kill cam. The Volkihar's feeding habit is almost opposite of that of the Sanguinare. The Volkihar gain Vampire Stages as they feed, but must feed more often to maintain it. The higher their Vampire Stage, the more they must feed to maintain that stage, going into a veritable feeding frenzy at Stage 4 with a very frequent need to kill and feed if they wish to maintain Stage 4. Volkihar can avoid being discovered as a vampire only when at Stage 1, their weakest.

Wild, feral Werewolves would spawn in the wild at night, and Werebears would be introduced as well, along with a Lycanthropy Perk Tree.

As someone previously mentioned, this might entwine with the rumored announcement for an MMO. Dawnguard contains two words that can be identified with The Elder Scrolls franchise, and I'm especially regarding the first term, Dawn, as it is associated with the name of Tamriel (Dawn's Beauty) itself - and no one here has mentioned that 'til now.
But I might be way over my head. It's just speculation, though.

Probably a DLC that involves the gaurds of Dawnstar.

One thing that was not included in Skyrim was that monster that shows up everytime you rest in the cell phone game entitled 'Dawnstar'.

Or this could be a new title, with little to nothing to do with the Elder Scrolls. Or Fall-Out.

Or it's just balogna.

How would you justify a new town appearing out of nowhere?

Maybe it was destroyed in the past centuries and subsequently abandoned like Labyrinthian (which had a different name, btw. It's mentioned in one of the books). I do wonder if Bethesda will include in one of the DLC the ability to rebuild Helgen in 'RavenRock style'

I would really hope that with an DLC the story would progress a little, so Helgen could be rebuild, Whiterun fixed (after Legion/Stormcloacks storyline). And maybe even in the next DLC the Moot would finally choose a new high king. That'd be sweet