A core part of the first game is how isolated you are though. I don't usually say this with survival crafters, but for me coop would take away more than it would provide
There is a mod that enables co-op for Subnautica. It was a pretty fun, but when I played it a few years ago, it didn't really add anything, it was just that you could have two people going through the story together and harvesting resources etc.
My thing about it is designing for both an isolated feel and a cohesive co-op may be impractical, and I'd prefer they focus on the part of it that makes it really stand out. There are also things like how do you progress world events, can either player trigger them, do they have to be in the same area together, etc which all adds design overhead.
I also want more good co-op games they're a lot of fun. Personally I don't think this is the right series to target for it though, but also idk what direction they're taking it so co-op may fit well with the new one.
I always enjoy a game solo at first, then do co op with friends. It was really fun solo. But I'd like to create a giant underwater farm with friends, and have various seamoths and other vehicles to use with buddies. The multiplayer mod works, it's just buggy as mods are.