I think Dawnguard will be more about investigating the Vampire's plans and stopping them (from the Dawnguard perspective) or setting the plans in motion (from the Vampire perspective) I'm speculating that the 'Missions' will be lone quests (IE, go here, take out this nest of vampires, find this clue, it leads to the next location, so on and so on) which I imagine will lead to a Showdown between you and the head Vampire (sorry, can't remember his name, too lazy to look it up). One thing about all TES games, they make you feel like a Hero, or Villian (I'm guessing you could end up as the head vampire if you choose that path).
I'll have you right on the Civil War quests. It didn't feel very personal. I hope Dawnguard does