I think the DLC will have several main features, building modular houses being one of them. Just like becoming a Vampire Lord is one of the main features in Dawnguard yet it offers a lot more than just the Vampire Lord form.
One wold hope.
I would be okay with the idea of them added a bunch small, new features like new enemies or spears or areas a couple times a month for a cheaper proce while they work on a bigger DLC, because that would keep me interested in the game.
As it stands now, I played Skyrim for around two months, got bored, then didn't touch it again until Dawnguard came out, which is not a process I find enjoyable.
If this just turns out to be Bethesda asking me for five or ten dollars so I can build a house in Skyrim, I would rather they have released it sooner, because it obviously would not be as taxing a feature to add as something like the Forgotten Vale or the Soul Cairn was.
I'll buy it, because I'm a completionist, but I think the methods of Bethesda's DLC releasing are effective, these things should come out more quickly and with more frequency.