Had a look at Below Zero. It doesn't interest me as much as the original, for various reasons.
X The original had a truly open space, the lack of scenery gives it a sense of massive size.
+ Below Zero has scenery in almost every direction that both blocks the view and player progression. It makes for a nice-looking horizon with mountains and glaciers, but at the same time it really cramps on the scale of the game and gives all-too easy landmarks to navigate by.
X While the original had a few artificial gates in place to guide the player's progression, it also played on its own open-worldness and allowed for surprising flexibility that rewarded smart players.
+ Below Zero seems to do away with all that by having even more artificial gates in place. Even in the first open underwater area I feel boxed in and waiting until the plot finally permits me to go elsewhere.
X The original suggested places for the player to go, and wouldn't be that bothered if the player just dicked around.
+ Below Zero commands the player to go there and do this in a seemingly exact order.
X The original didn't have fixed characters, the protagonist is a Tabula Rasa for people to build upon themselves. This makes for better immersion as players feel as if they're truly there*.
+ Below Zero has fixed characters. This would be cool if they were interesting, but so far the protagonist is a British skank, her twin sister (who replaces the computer from the original) is a social media 'influencer' and the third character (won't spoil it) is Wheatley from Portal 2 (oh wait, guess I did spoiler it). In fact, with these characters constantly droning on in the player's ears one can't help but feel like this is Portal 2. Or a walking simulator. Ugh.
X The original played on the theme of loneliness, solitude and being shipwrecked in the middle of nowhere with nothing but an emotionless PDA to talk to. It was a rare approach to a video game and was one of the reasons why Subnautica stood out.
+ Below Zero can't seem to go two minutes without someone blabbering in the player's ear. Because of that there's no sense of loneliness and solitude, and Below Zero feels like just another 'narrative' FPS game. We've seen plenty of those over the past 20 years. Yawn.
X The original allowed the player to scan almost everything to learn more about the environment.
+ Below Zero does away with all that, the player can only scan precursor tech, relevant gear and lifeforms. No more scanning plants and other 'trivialities'.
There are probably other points I'm missing, but the point is the devs seem to have done away with everything that made Subnautica unique, and replaced it with stuff that can be found in every other FPS game out there to the point that the game now feels bland. The only clear improvement I've seen that Below Zero has over the original is weather patterns.
While the normal caveat applies that this is all based on its current Early Access status, keep in mind that I'm talking about basic game design decisions here. The devs would have to go back to the drawing board to change some of this stuff. Let's wait and see what happens.
*INB4 the raving SJWs droning on about how the protagonist of the original is a white male and that not everyone can associate with that. The blame for the white male protagonist in Subnautica can only be put on the shoulders of the developers. During its development the devs were often asked to allow for more and varied character models, and they even promised they'd do so. However their own incompetence got the better of them so everyone got stuck with the single low-effort character model. But at the end of the day it's still easy to look past all that and just have conjure up their own headcanon as to who's trying to survive down on 4546B.