"Hearthfire" is a book within the game. Its also the 9th month in the calandar within the game. If you go to the skyrim wiki, it lists a few annual events that happen during the month of "hearthfire."
"1st of Hearthfire: The Rites of Moawita practiced by the Psijic Order"
I'm betting that the 2nd skyrim DLC will involve the psijic order, the summerset isles and the Thalmor (nazis).
"The Psijic Order removes the Eye of Magnus from the Hall of the Elements, promising to safeguard it from the Thalmor."
Info on the Thalmor from the wiki:
The Thalmor, as of the Fourth Era, is the governmental representation of the Second Aldmeri Dominion, the union of Valenwood and the Summerset Isles. As explained by Delphine, the members of the Thalmor are "elven supremacists" who seek to end the Empire and eradicate the worship of Talos
As a branch of the Aldmeri Dominion, the Thalmor seeks to unify the provinces of Tamriel through one-government leadership, with Elves holding all places of power. Deeply offended by Tiber Septim's victory in Valenwood, members of the Thalmor have sworn to root out Talos worship in the Empire. The White-Gold Concordat, effectively ending the war in Cyrodiil, gave the Thalmor legal authority to eradicate Talos worship, although many devout believers, including Ulfric Stormcloak and Elisif the Fair relented, either openly or subversively.
I think what will happen is the Thalmor will seek to take over the world, steal the eye of magnus (Raiders anyone?) , and it will be our job to travel to the summerset isles to retrieve it and stop them.
Obviously it will be about werewolves. Dawnguard is all about vampires, so it's only fair that the next DLC be about werewolves. The name even hints at it, dogs love laying in front of the hearth while there's a fire going. Pretty obvious