Considering that the vampires of Dawnguard seem to be connected to Molag Bal according to some, is it possible that the Dawnguards are connected to Azura?
Azura is a "good" deadric princess, in contrary to Molag Bal. She is the deadric princess of dawn and dusk, and maybe that has something to do with the dawnguard? Maybe they are dedicated to serve Azura? Considering that the vampires want to "end the terror of the sun", this may be something Azura wants to stop.
Do you think this may be plausible?