Oh, nice, a Dawnguard speculation thread in the lore forums.
So I can submit my favourite theory to the lore buffs and see what they think of it.
The datamined files were, among others, about vampires and Falmers... Well...
http://www.gamesas.com/topic/1371845-dawnguard-and-future-dlc-discussion-thread-26/page__view__findpost__p__20743880 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 disappearance of the Deep Elves.
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...
Azura's realm, Moonshadow, is said to be so beautiful that it turns the ones who visit it either blind or mad. Well. Maybe the Dovahkiin's glowing eyes on the announcement picture 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 seem 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 thus was seen by his or her followers as the wielder of "Twin Lamps"? This would explain the very origin of their name. 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.
Is it too far-fetched to be plausible?