I love Subnautica have played it all the way through, with and without mods, many times. However there are a handful of things I can think of that would improve the gameplay in a variety of ways.
Speed limitations have to exist on vehicles due to the engine not being able to handle loadzones well enough. Upgrades for land speed on land for the prawn suit are fine because it moves poorly on land, but …
Great job devs, keep up the good work. I've recently been replaying subnautica for the 5th time so your work has not gone un-noticed xD. I don't replay games often, but this game always leaves me wanting more when I complete it.