It depends on if Obsidian is available to do it.
It also depends on if their studio can handle this kind of triple-a development.
And then it depends on if Bethesda will give them enough damn time to finish the game.
Finally, it depends on if they are at good terms, and considering Bethesda snuffed the royalties from Obsidian because of one measly metacritic point, something that Bethesda was responsible for in the first place, and considering that Obsidian can now crowdfund games if they so choose, I doubt they'll be head over heels to want to create another Fallout game under Bethesda's conditions.
We might get a spin-off, but InXile is too small and Obsidian might not be on best terms with them. So what then, Interplay? Yeah that worked out well with Fallout Online.
If we get a spin-off it'll probably be by a studio that doesn't have any former Fallout developers.