Update: A new day, a new tweet.
https://t.co/UasrXtJf6C pic.twitter.com/dlKmzsyBjUJanuary 19, 2017
The "Leaden Key" reference gives away that all of this is something to do with Pillars of Eternity: The Leaden Key, as explained in the Pillars of Eternity Wiki, "is a secretive organization dedicated to the goddess Woedica and the preservation of the secrets of the gods." It has existed for thousands of years, led by an apparently-immortal grandmaster named Thaos.
That still leaves a lot of room for questions about what exactly it could be, but barring some bizarre crossover, it is quite clearly not a new, Obsidian-authored Fallout.
Obsidian Entertainment, the maker of Fallout: New Vegas, Pillars of Eternity, Tyranny, and a bunch of other stuff, tweeted something rather odd today: An image of a pipe and a stalk of wheat, accompanied by a message about the return of gods and lost dreams. It also put the same image up on the front page of its website. What could it mean?
https://t.co/yYtvhfc4jV pic.twitter.com/8dCj0tX2bGJanuary 18, 2017
A follow-up tweet, encouraging followers to "talk amongst yourselves," included a link to an Obsidian forum for something called "Project Louisiana." It currently contains two threads, one positing Fallout 4: New New Orleans, and the other insisting that it's obviously Pillars of Eternity 2.
Neither theory has any evidence to support it, but you will recall that there was talk about a Fallout: New Orleans last summer, fueled by a European Union trademark filing. But Obsidian quickly shot down that rumor, and the filing now appears to be gone.
I've reached out to Obsidian for more information, which to be perfectly honest I don't expect will be forthcoming. If it is, or if there are further developments elsewhere, I'll let you know.