So, in an ideal version of your game, I could imagine the following:
* An aesthetic evolution of the submarine when it is upgraded (very slightly, it wouldn't be a question of modifying its design but more of adding a few small details as you go along).
* A codex of analyzed species, allowing you to immerse yourself in Europa's universe and give free rein to your imagination concerning these life forms.
* Why not add a mystery to discover? For example, discovering the remnants of unknown technologies when you go down deep. This could be an opportunity to play with lights in the dark depths. Your light management is extremely well done, so I'd use it more (while staying dark).
This technological life-form could take sounds from the depths. The muffled sounds are also very well integrated.
(I'm not in the habit of lavishing praise on you, but I have to admit that your game left me on my ass!)
* Well, the animation at the beginning is a bit long, but the other comments have pointed that out to you.
If I come up with any other ideas, I'll let you know. I congratulate your team again, the sound design is perfect, the game play fluid, the inertia of the submarine and the art direction... You'll go far!
As for my game, it will probably be of little interest compared with yours. I'm more proud of the previous Brackey gamejam, which I continued to develop. A title in space but much more arcade-like. I'm not much of an artist, my skills are more mathematical! Here's a video of where I'm at: