The 3 rumors I heard about Hearthfire are as follows:
Magic Base
Building Your Own House
And Zombies.
Now.. Here is my conclusion of how all three of these rumors can be true and connected, in a way... In Tamriel, there is a month known as Hearthfire and there is a holiday on this month which is called "Tales and Tallows", if I'm correct. The holiday is for Necromancers and to celebrate the magic of resurrecting the Undead.. With this magic being legal now, my thought is that there is a group of Necromancers that are working to achieve something powerful this year and will be doing so on this holiday. What would they do? My best guess will be that they will try to bring back a very powerful Necromancer, or mage, from the dead as a Lich OR their leader will perform the ritual to become a Lich himself. They will most likely succeed and this Lich will be raising an undead army until it is time to attack... The actually questline of the DLC will begin when the Lich made its first attack and it shall go from here...
This explains the whole "Zombie" rumor but what about Magic or the House? Well.. The Lich will be a powerful magic user and will prolly use a lot of spells already in game, as well as new ones.. So they may update the spells and add the new ones to help make the battle more of a fight, and so on... So therefore, this would be a good time for a magic update.. Now for the house.. Well.. They did say something about a Skeleton Butler, no? So.. What if they add the option of building your own house in this DLC and doing a quest, or part of the questline, will unlock the Skeleton Butler... So.. There you go.. My conclusion..
Oh! I'm gonna throw this out there.. They could also turn this into a Conjuration-Base DLC where the Necromancers will be fighting with the Conjurers (Daedra Summoners) and you get to pick a side... Could add more Daedra finally, eh?
~Edit~
About the whole "Eye of Magnus" stuff people been saying and wanting... I'm gonna bet that Skyrim's BIG DLC will be base on that which will most likely be the last DLC, not Hearthfire.