Any roadmap?
You have good system base to merge sandbox with a singleplayer campaign. Rather than being formulaic, by-the-rails mission based, you can fuse it with sandbox. What do I mean?
Well, start story campaign with a ship crash. Then choose which character you play as. You start (either in a fixed position, or somewhere random) and start exploring. From then on everything is more sandbox, but still structured. You'd obviously have basic objectives (for example, establish a base, gather suvivors, etc.. ) - but HOW you go about it is up to you.
NPCs you run into you could befriend, kill or enslave. Other NPCs could make their own small camps and form groups. Basic faction relation mechanics. Heck ,even factions and groups might be dynamic. Would be fun.
I know some might be upset with this, because obviously some people would kill/genocide certain NPC's in at leat one of their playtrough, but that is a non-issue.