Thanks to job listings we've previously reported about, we know that Naughty Dog is working on a multiplayer game that seems to be built on live service foundations. It is widely believed to be related to The Last of Us: Part II, which launched entirely without multiplayer.
It does however seem like the project has grown a lot. Thanks to a new job listing for a level / environment designer (multiplayer), we now know it is a standalone game, as the description explains that it is "the studio's first standalone multiplayer game". They also seem to have high ambitions:
"We're seeking to bring the same level of ambition and quality of our signature, story-driven games to this unique multiplayer project."
When this project will be revealed is unknown, but hopefully it isn't too far off.
Thanks GamingBolt