Damn, that is some impressive work by these modders.
Bethesda have been drumming up their just released Fallout 4: Far Harbor DLC for quite a while, now- and for good reason. With Far Harbor being the single biggest bit of post launch content that Bethesda have worked on ever, the new DLC pack is a veritable expansion pack, or a game unto its own.
However, even Far Harbor will pale next to the glory of Cascadia, a new mod for Fallout 4, which will be a little bit smaller than Fallout 3, but far bigger than Far Harbor. Cascadia, which will be an all new story set a few decades after Fallout 4 and beyond Boston, has a team of 18 people working on it, and it will have more locations and quests than any DLC released by Bethesda yet.
It sounds very ambitious, and what is more, it may not be restricted to just the PC, either- in addition to Nexus Mods, the team is also looking at publishing the mod for Bethesda.net, which would mean it would be potentially coming to PS4 and Xbox One too- although naturally, that is not something they can commit to yet.