I bet it is a single player.
a) SD should do a single player, just so they can tick it of the list. It’s a natural evolution for a company with ambitions.
b) Some of their key writers have been quite silent for a relatively long time. I think they are focusing on this project.
So, what will it be? Surely a shooter. Will it have a multiplayer component? Yes, I think so. I mean, they know that part of game development. But what will be game be about? What is the story, what is its setting in time and geography?
Given the huge risk, I guess SD is playing safe, since it is their first single player. So, forget space, squirrels, and portals. It will be an easy-to-understand, easy-to-remember backstory, and an intuitive game. Pick up you gun and play. Still, in true SD style, it will have depth and replayability. And it will be something completely new, i.e. a new intellectual property, owned by the guys. (In other words, no sequals or identical clones of previous games.)
For the protocol: I am just brainstorming here. I have no facts or other evidence to justify my claims above.