"Subnautica in space" as in "outer space" or "on an alien planet"? Because outer space is kinda empty. Probably wouldn't make for as lively a backdrop as under an ocean
The way Breathedge got around it initially is the starting area is a ship crash, so you collect broken bits of the ship(s) (which includes water and food).
But space is vast. Why couldn't there be space fauna? Or a way to travel to nearby system planets? Its fiction, after all. We don't need to be constrained by reality.
If you get a spaceship and can just go wherever, that sounds to me exactly like NMS. I think part of what makes Subnautica what it is is the constrained resources of the environment, and that feeling of being stranded. NMS lets you just bum around space forever, which is fun, but you don't really feel that need for survival like you do in Subnautica.
NMS is survival in space, insomuch as planets are in space and you can fly around in a ship, but you start on a planet. I was thinking more like having to survive in space by building the ship in space, building a station in space, etc. Space would be your primary sandbox, rather than planets (at least initially).
The normal NMS experience isn't quite what I'm envisioning. Maybe if you started on one of those abandoned freighters, though...