I believe a decent addition to the game would be a new kind of structure, The Keep. The keep would be designed specifically for defensive purposes. The reasoning behind it is to offer people who get trapped in their own villages/claims a chance to defend themselves, I'll explain how further down.
The keep would work the similar to the current houses, a ground floor, a basement addition, an upstairs area, and then a third floor which would be open to the outside world. To keep the structure from being too over-powered, the inside should only be 5x5 tiles in size, to keep them from storing multiple hearthfires and loads of valuables.
There would be two main advantages to this structure, first of all the door would work similar to a gate, it could be open/closed by anyone with a key, making it very ideal to store any goodies as well as hearthfires. The second advantage would be when a person is standing on the third open floor, they would receive a boost in ranged attacks' accuracy and marksmanship, so one could defend effectively. Obviously the attacker on the ground level would have their accuracy and marksmanship reduced only when attacking the defender. As explained in the link below the sole purpose of the keep is a last resort for a castle that was under attack.
Now, to the creation idea. The keep should be very expensive to create considering how effective it would be, it would obviously require a LOT of stone, perhaps stone boulders themselves could be part of the construction process. Other requirements could be a significant amount of steel, bricks, planks, blocks of wood, etc. My suggestion would be: 1600 Stones, 80 Steel, 400 planks, 500 blocks of wood. Destruction of the keep would require a ram and a significant amount of strength to destroy, making it an effective yet costly means of defense.
Now to the part that ties it together, the keep would require a village claim with at least 300k authority daily, if the authority begins to drop the keep begins to rot, which would mean the village would HAVE to be active in order to keep their defenses. There can only be one keep per village, to prevent large vaults, and as far as the hearth vault issue goes hearth fires could not be created within the keep.
Now some criticism would be nice, whether it be good or bad. I did a quick search and did not find any threads solely about the creation of this structure.
Edit: For some basic historical information: http://en.wikipedia.org/wiki/Keep