Pragmatic Brain Says: "New" Game. Because if it's an Expansion, it would fall under the season pass, so less money.
I mean, I have the season pass, so I'd be 110% up for a new expansion, but.... I doubt it.
Yeah as more details have emerged its obviously a spinoff game set in the Fallout universe. It's sounding like it will fall somewhere on the spectrum between ARK/Rust and Battle Royale.