I really would like to see another RP/Questing focused guild like Dark Brotherhood or Thieves Guild. I think Dawnguard would fit that role perfectly.
In Skyrim, the Dawnguard had been gone for a LONG time. Not much is known about the formation or the heyday of their organization. If the lore fits, i think ESO would make the perfect timeline to show all the early days of Dawnguard. Maybe your character could be part of the original members that helps form it? At the very least I would love to see some new type of quest where you sent out on bounty missions to kill beasts and vampires in scary locales. Maybe introduce some new kind of mechanic where you have to actually track down your target into certain locations. I dont know, im just rambling. What doe you guys think of this?