It will be a completely separate adventure from the main game, and you won't even be able to visit Commonwealth, so you'll need to create a new character.
There are plans to release the mod on all platforms, however the team currently can't guarantee it will work on consoles. There's no set release date yet, as it is made by volunteers who just work on it in their spare time.
While the wait for Cascadia begins, you do have the official expansions to check out. IGN's review of Far Harbor says it contains some of the best Fallout 4 quests we've seen yet. Matt Porter is a freelance writer based in London. Make sure to visit what he thinks is the best website in the world, but is actually just his Twitter page.