Fallout: Cascadia is going to be a fan expansion set in Seattle. It uses the Fallout 4 engine, but it seems to be drawing a lot of intellectual ideas from Fallout 3 and before. In particular they want to bring back S.P.E.C.I.A.L. (stat) checks, which is something that was only mildly used in Fallout 4 – and I’m actually a little surprised about that.
There’s a big team on this, so hopefully we’ll have another Falskaar (a similar project for Skyrim) on our hands. Eventually. This is probably going to be a while.
PS: I certainly hope that the Enclave is involved, somehow. I’m still convinced that they were originally going to be in Fallout 4, but then got turned into the Gunners.