Subnautica is one of the better single player survival games, offering broad exploration across a vast ocean with various biomes and dangerous creatures to avoid. It's terrifying at points, especially since it takes a passive approach to interacting with the local wildlife. It's since been expanded with an excellent Below Zero DLC which is still under development, but the developers at Unknown Worlds would love to do a full sequel. They might even include multiplayer.
"We knew right away that we weren’t doing multiplayer with Below Zero," art director Cory Strader told PCGamesn in an interview. "It was initially going to be an expansion pack, it was a much shorter timeline. So we knew that was totally out of the scope. I would imagine we would be rethinking those things, at least the multiplayer side of things, if we do a future Subnautica game."
Official multiplayer support was never planned for the first Subnautica, although VR support was. Modders have managed to make both possible in a limited capacity, but official support for both (particularly multiplayer gameplay) are some of the most requested features of future Subnautica games.
Unknown Worlds is also reportedly working on a secret game, with the Below Zero team moving over to that venture once the DLC is completed sometime this Spring.