I wonder if it's possible to create a streamlined platform for this kind of work. Like others mentioned being able to start at an outline stage and then move on to fleshing the parts out would be interesting. I could see the outline being its own repo and each "unit"(probably chapters) get their own repo and maintainer. Nightly builds could be conducted to merge the work in the repos into a coherent whole, reconciling missing parts with their outline bullet points. An interface over the "build" to make quick commits and PRs would be nice too.

Additionally a community style guide and "story bible" would probably be in order. I believe a lot of writing projects which involve a large number of writers like video games and TV series do things like this. On an unrelated note, are there any tools to enforce natural language writing style compliance? Like code linters for human text?