It's not a Fallout battle royale. That was never a valid rumor and was squashed quite early.
Possibly it's a Fallout survival RPG, which would be abso-frigging-lutely fantastic, if they do it right (i.e. take some cues from Conan Exiles). I consider this the most likely thing, to be honest, especially as the rumors are that the base-building in FO4 was sort of a test-run for this.
Also possible it's a glorified Fallout Shelter, with online play and fancy graphics. I am uninterested, if so. I have zero interest in a vault sim game.
Remotely possible it's a Fallout MMO. That would be pretty good, if they apply what they've learned from ESO, which has evolved over the last 4 years to being one of the best MMOs on the market.
Or it could just be another Fallout RPG, in line with FO4 but not a "full FO5", and featuring online co-op, rather than the full multiplayer approach of survival games or mmos.
Regardless, what is known is that it's developed by their Austin studio, which was formerly working on Battlecry. And Fallout 5 is still in the works, with this just being a sort of side thing. A Fallout 4.5, if you will.