Nah. They wouldn't need to remake the entire game, maybe the story will be changed a little (Two humans on the planet instead of one, bigger rocket, etc). They have all the sprites, etc, so I don't see why they would need to remake it.
I believe since Multiplayer isn't their first priority, they'll finish the game first, release it, then add multiplayer later on to attract players. Of course, 1-2 or 1-3 would be a lot.. I think 10 should be the max.
Not to mention accoutns would need to be created inside Subnautica to make everyone recognizable. Looks may also vary, but being able to customize your own character will be nice.
You know what you can do to speed up develoupment?
Dream and hope a little.