I'd heartily support a Colonist book.
Making it essentially the citizen career sourcebook, you've got space for world/environment/map building and some guidance on how to fit skill development to an environment - whilst the homeworld skills provide some ability to fit your skills to the environment, on a nascent desert world/ocean world/ice world/void world/whatever you're going to want some guidance on putting together essentially 'custom careers' like Dolphin Farmer, Void Construction Worker, etc.
Making it essentially the citizen career sourcebook, you've got space for world/environment/map building and some guidance on how to fit skill development to an environment - whilst the homeworld skills provide some ability to fit your skills to the environment, on a nascent desert world/ocean world/ice world/void world/whatever you're going to want some guidance on putting together essentially 'custom careers' like Dolphin Farmer, Void Construction Worker, etc.