A minor issue with both of the first subnauticas is players can miss out on a unlock and struggle to find that one piece of a recipe laying around somewhere, they could for example have the Cyclops unlocked but still not have found the last piece needed for the PRAWN suit or similar.
I think for Subnautica 2, you could make the research tree a little more interesting and evolved, for example, break each type of fragment down to a type, such as “Gear”, “Vehicles”, “Base” and finding fragments of each time moves you towards that tier instead.
As an example, starting fresh you have nothing, your first vehicle fragments unlock the arm attachments, the next ones unlock the Tadpole, then you progress through to the next vehicles.
This would help with immersion, as it would stop players needing to exit the game and go check wiki’s to go find that elusive part they can’t find anywhere, because they missed a fragment one time in a location they thought they had cleared.
You could do it for upgrades too, first upgrades you find for the tadpole, then the next vehicle, base… etc.