I believe that in a later patch the Dawnguard will be formed (If not already is being formed.)
Sometime in the Second Era the Fort Dawnguard was built to contain the Jarl of Riften's son, who became a vampire (Who later was killed.) Also as Tamriel came under attack by Coldharbour, by hordes of Vampires, Daemora, etc, Its possible now is the time when all this begun.

I am not entirely sure how they could make a whole patch out of it, so maybe an Adventure Zone?

If anyone else has anything to add or there is flaws in my logic/lore, please comment