Fallout Cascadia is a new mod that is promising to create what’s essentially a brand new game—using Fallout 4 as a foundation—set in and around Seattle.
The plan is to have the mod kick in as a completely separate adventure to the main game, so you wouldn’t even create a character or see Fallout 4's intro movie before being whisked away to Cascadia. Nor could you travel between Seattle and Boston; this would be entirely its own thing.
It’s set “several decades” after the events of the actual Fallout games, and while it will focus on the city of Seattle itself, there’ll also be other locations “including Mercer Island, Redmond, Bellevue, forested areas and airports.”
Some of the folks involved in this have also worked on Skywind, the ambitious attempt to remake Morrowind using Skyrim’s engine. And while that’s enough to give cause for concern—that thing’s been in the pipeline for a long time—one of the developers involved says Cascadia (which is using many existing assets) will take a lot less work than Skywind, which has had 70% of its stuff made from scratch.
Cascadia’s team are hoping to get the mod out on at least PC, though they’re aiming (but not promising) a launch on consoles as well.