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The Expansion Pack is an upcoming paid DLC for Subnautica. The full release date is currently planned for early 2019.
Currently all that is known is that the Arctic Biome will be a large part of the game, possibly the entire map. It is also known that a weather system will be featured.
Based on a tweet from game director Charlie Cleveland, it is likely that the Arctic expansion will be a separate game mode as he has stated that the protagonist will be a currently unnamed female rather than Ryley Robinson.
Features that have been confirmed through various means.