Well, since this thread is new, I'll try to put together all our speculation so far. I know I am biased by my own opinions, so if I forget something, please tell it.
All the official, confirmed information we have about the first DLC is the image on Bethblog and its name : Dawnguard.
Besides, leaked data from the last patch features a Snow Elf Prince, vampire feeding animations, crossbows and a mysterious "RF", suggesting that all will appear in the first DLC. Maybe a "Riekling Follower" or a "Restored Falmer"? But maybe the Devs have intentionally let those data leak in order to mislead us, and give us a real surprise?
And finally, we all saw the content from the Game Jam.
Now, let's see what the theories built so far are.
1. Molag Bal, Azura, the Twin Lamps. In the last thread, railtracer realized that the connection between Falmers and vampires could be Molag Bal. Molag Bal created the first vampire, Lamae Beolfag, the Blood-Matron. Moreover, Molag Bal is the Prince of Domination and Enslavement of the Mortals. Therefore, he might be involved in the narcotic enslavement of the Falmers by the Dwemers and its perpetuation even after the Disapearnce of the Deep Ones.
Azura's followers have already fought against vampires in the past, the specific example of Dratik appeared in Oblivion. Moreover, Azura is the Prince of Dusk and Dawn. Thus, Dawnguard could be the name of a faction of her worshippers, who struggles against Molag Bal. Let's expand a little railtracer's ideas...
Alas, Moonshadow is said to be so beautiful that it turns the ones who visit it blind or mad. Well. Maybe the Dovahkiin's glowing eyes are in fact an outer sign of the change which happened after his coming to Moonshadow? Maybe other people visited it in the past, and came back with such eyes, and were then wrongly considered blind? Or maybe those eyes are a blessing from Azura, allowing its wielder to walk into Moonshadow without harm?
The natural opponents of the Prince of Enslavement see to be the Twin Lamps encountered in Morrowind. Now, would it be possible that its name originate in such a blessing of Azura, that the founder of this order had two glowing eyes, and was seen by his or her followers as the wielder of "Twin Lamps"? Could the Twin Lamps of Morrowind be actually a subfaction of a larger Dawnguard?
So a new hero, blessed by the "Twin Lamps", could have to free the Falmer from their perpetuated enslavement, restore them, and then fight against the worshippers of Bal's creature, Lamae Beolfag.
(Besides, if Lamae is featured in a DLC, or her followers, I hope we'll also meet the Shrieking Winds.)
2. An insular DLC.Since the Snow Prince sepulchar was located in Solstheim, and since the East Empire Company is featured in Skyrim, one could consider that we'll have to go back there.
Besides, the flying vampires featured in the DLC don't look like the Volkhiars described in the lore, who passwall through the ice of frozen haunted lakes. They could arise from another strain of porphyric hemophilia. So where would another clan of Vampire, possibly worshipping Lamae Beolfag, be located? Maybe on the island of Olenveld, which was Talos' personal graveyard.
Finally, some Falmers could have fled to Roscrea instead of seeking shelter by the Dwemers, and they could be lead by their new Prince and claim their lands back, making Roscrea a possible location for the DLC? Moreover, the existence of Roscrea is acknowledged, but nothing is known about it, so the Devs would be entirely free to build it hte way they want.
If the three islands are featured, one could also hope to visit the Castles of Glass and Coral of the Dreughs, in the Sea of Ghosts, which would please the many ones who want to visit places drastically different from Skyrim.
3. Meridia.Another hypothesis is that the DLC would be focused on Meridia. Arguments in that direction is that Meridia's blade is called Dawnbreaker, that shines like the Dovahkiin's eyes, and that among the servants of Meridia are the Aurorans. The English meaning of the latin word "Aurora" is "Dawn".
4. The Dream of the Thalmor. Since the Thalmor wants to create a new Dawn Era, it has also been hypothetized that the DLC could revolve around the Thalmor. Maybe as a continuation of the Civil War.
5. The Guardians of Dawnstar. Dawnstar was once besieged by the Ice Tribe, who were possibly Falmers. Maybe the Dawnguard is some faction created to prevent it from happening again?
6. The Dawn is what rises from the East. Maybe the Dawn is a metaphor for "something rising from the East"? Then either from Morrowind or Akavir?
We know that the Tsaesci already invaded Tamriel, and that some akaviri magic involves dusk and dawn, as is proven by the blade called Dawnfang.
Besides, the Kamal already tried to invade Morrowind. Maybe Rieklings and Grahls are refugees of the disaster of Vvardenfell? Also, one could think that the Ice Tribes weren't Falmers but came from Kamal? After all, a Gehenoth looks more like a Snow Demon than like a Snow Elf, and the existence of an Ice Tribe Haven suggest that the Ice Tribes came from the seas. Also, some Snow Elves could have fled to Kamal rather than seeking help from the Dwemers. Now, they could come back and claim their old lands?
7. Werebears. The Dovahkiin of this new picture looks more tired than before, and has more facial hair. This, and the glowing eyes suggests that he could be ill, or cursed, or something between the two. We saw from the Game Jam video that the Devs have thought to introduce werebears, and indeed, werebears are mentioned in the lore. Maybe the Dovahkiin's facial hair is grwoing because he is about to be transformed into a werebear?
8. Nirnroot. We know that the ancient Nirnroot had a yellow glow. Does it have anything to do with the Dovahkiin's glowing eyes?
9. Dawn's BeautyTamriel means "Dawn's beauty" So, could the DLC feature a new threat to the whole of Tamriel?
10. Mythic Dawn. Dawnguard could have something to do with the Mythic Dawn. There is a museum of the Mythic Dawn in Dawnstar.
11. Pelinal, Morihaus. We know that Morihaus wath "the Breath of Kyne" and was associated to the thu'ums. We know that Pelinal came from the future, that he drank the blood of his ennemies in victory ("Vampire feeding animations"...), that his hand was shining (as the Dovahkiin's eyes?) and that he missed the Blessing of Talos. So, could the Dovahkiin become Pelinal? Could he miss Talos' blessing due to the Thalmor's prohibition?
So, feel free to add the ones I have forgotten, as well as new ones, and new suggestions.
» Wed Jun 13, 2012 11:24 pm 
a bit of patience and a positive attitude perhaps would be more satisfactory 
Hope they keep what they promised us,That the DLCs will feel more like big expansions..