It does sound like a really fun idea. There are a few convenient chokepoints for this too, so mechanically, it could work.

I do doubt about the purpose of houses as "forts" though, or how crafters and gatherers could contribute except early on in the preparatory phases (like making cannons).

Otherwise, it sounds like a great new PvE game. I can see this fitting easily in to the lore too. Maybe the devs make a new area with an Ishgardian fort or maybe Idyllshire that you defend against monsters and dragons. I do feel like it'd take on some aspects from the Steps of Faith though, which may not be bad.