Unknown Worlds Entertainment is currently making another Subnautica game. They are publicly referring to it as "Subnautica 2." So apparently they consider Below Zero to be a standalone expansion to Subnautica a bit like Half-Life: Opposing Force rather than a full sequel.

A few things that I have heard the developers confirm about this new game:

• It will take place in the same universe as the first two games.
• It will not take place on planet 4546B
• There will be swimming in it
• There will be submarines in it. Plural.
• It will be co-op capable. If I understand what they've communicated right, it is going to be a single-player game that will have a "join game" button so you can invite a small number of buddies to join you.
• From the screenshots they've shown, there's going to be colorful ocean wildlife in it.
• According to the Wiki, one of the original concept artists and the composer for Below Zero's soundtrack are working on it.

They're playing a lot of details close to the chest for now, very few gameplay or story concepts have been discussed. It is very likely going to be an ocean survival game with a nonzero chance of having your submarine bitten off by a 300 foot scream eel.