Does Dawnguard require the player to have beaten the game? Or perhaps is there a level requirement (like Bloodmoon where a new character would get slaughtered)?
*spoilers* * * * * * The trailer mentioned the vampires want an elder scroll to carry out their plan, and I know there is an elder scroll in the main storyline of Skyrim so that made me think maybe they are after THAT elder scroll and thus Dawnguard is supposed to take place chronologically after that quest, thus requiring the player to have completed that quest before starting Dawnguard.
I heard that it's designed for level ten, but i don't have the source. Bloodmoon was for morrowind (which you already know) and was difficult for low level characters because the games enemies were a constant level and type in each region. Since skyrim enemies level with the player, I doubt it will be too difficult for any level character.