I have a pretty good gut feeling that Dawnguard will be a werewolf clan of some sorts. And the player will choose whether to join up with them and become a more powerful werewolf or join up with the vampires and do the same. Now this theory does leave out the idea that the player can go through the story without choosing one or the other, perhaps there's something that we don't know, maybe there's a third "choice." Who knows. Here's what I do know...
1. In the beta test sign-ups one of the questions, as I'm sure most on here know is whether or not you play as a werewolf or a vampire. Bethesda would not ask this question if there were not ties to vampires and werewolves in Dawnguard. They want to be sure that there are some of each in the beta testing. I think this is the best indication that werewolves will get some love in Dawnguard.
2. Vsions has said that the future looks good for werewolves and gave a little "wink" in one of the other threads that was discussing the possibility of werewolves getting some improvements or attention in the DLC (or at least future DLCs).
3. Bethesda LOVES to try and keep secrets to surprise its audiences, which I appreciate. However, most of the time these are brought to light before they can be delivered the way Bethesda intends them, as surprises. Werewolves in general are a great example. Lets think back prior to release of Skyrim.
There were a TON of questions on whether or not werewolves would be in the game and we got the typical "we're not talking about that right now," answer from Bethsda over and over. However, the question was finally answered in one of the Q&A threads on this forum prior to Skyrim's release in which the answer was given. No. Werewolves will not be in Skyrim? Really? Bethesda straight-up lied about this, which I find pretty cool and funny. To our surprise and pleasure we quickly found out that werewolves, in fact, were in the game. And that' they'd be playable. So now here we are again...
I think Bethesda is wanting to deliver the surprise that they were unable to hide in the original release. This DLC will be vampires vs werewolves. The more I think about it, the more I believe its true. The Dawnguard have to have some kind of power, or something special about them. We'll know for sure soon enough.

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