In that case you should have an artifact to point to that describes the plan at a higher level. So you send a diff with a newly expanded role for some character

In that case you should have an artifact to point to that describes the plan at a higher level. So you send a diff with a newly expanded role for some character, the summary of the diff says, "Refactor chapter 1 to prominently feature Character", then the detailed description says, "Character will have an arc that culminates in Conflict in Act 2 and Resolution in Act 3. See {link} for more details on my current thoughts on where Character is heading." Then this new prose in chapter 1 also has to be good when taken on its own, no spelling or grammar errors and a generally nice sound to it. Then I would consider such a diff eminently mergeable. That is to say, a change doesn't have to be perfect on its own, but the direction should be clear, and it should be locally well constructed.