May I join?
I have an idea. the cave can have vampire slavers. They capture people and sell them to whoever has the gold(mostly other vampires). then you guys can go free the people and kill some vamps. after that you can torture the leader and he'll tell you about a huge Organization involving all of Skyrim.
The huge vampire organization can involve the dawnguard,(No spoilers of course). The organization will have key shareholders in every city. You would have to hunt them down and kill/capture them. Once you take out all of the share holders the organization will grow weak due to the Lack of Blood/gold.
Once they grow weak you guys locate the headquarters.(I'm thinking of either a cold island with a vampire village on it, or a cave with starving vampires.) Once you locate the headquarters, you acquire some gear and go vamp slaying.
Oh and if you want to bring the Legion/Stormcloaks in this that would be cool.
If I do join, I'm thinking my character would be imprisoned by the vampires. You guys come and save everybody then I tell you everything I just described. Since I was trapped there I overheard a lot of plans.