Unknown Worlds, the developer behind the Subnautica series has revealed that a new game in that very same series is in fact in the works. Revealed in a job posting, it's noted that it's looking for a Senior Narrative Designer to come and "help shape the next game in the Subnautica universe".
As this is simply a job posting, it doesn't provide all too many details on this upcoming game. It is mentioned however, that the applicant will be looking to "help establish the narrative direction for a beloved franchise," and will be "defining the history and lore of a new science fiction world and its alien inhabitants".
Similarly, as you can probably infer from the fact that Unknown Worlds is still in the recruitment process, there's no word on a release window or time frame for the game, but the repeated mentions of a game that is "early in development" means that it likely won't be coming for a couple of years at the earliest.