Well, from a story perspective we got two joinable factions and a third independent one. Vampires, Dawnguard, and Falmer respectively. Vampires want to block the sun so they no longer have to worry about their weakness to it. The Dawnguard, given their name, must already know about the threat to the sun. Otherwise they'd just call themselves vampire hunters or something, so I get the feeling they have a larger history that we just haven't known about - secret orders and all that. And the Falmer want to distroy the serface world. Given this, one could take a few stabs at the story. Vampires and Falmer join up, leaving the Dawnguard to stop both factions. Vampire and Dawnguard fight gets broken up by falmer trying to do more than just block the sun, forcing both sides to team up.
Stuff like that.
But based on such a name as Dawnguard, I just get a sense that they have a history. Other things want to distroy daylight, ya know? it would be interesting if the writers nip-tuck a few well known icons into the group, so it's history feels more real and less sudden.