More powerful editable regions (within protected docs)
This idea is based on Macromedia Contribute (from 20 years ago). It was a decent tool for collaborative content management in its day.
Imagine that you're a secretary, tasked with compiling the annual report. The report summarizes dozens of different projects and each project manager is responsible for authoring a project sub-report.
Rather than compile dozens of reports into a final report, you create a single Word doc, define editable regions within it, assign each region to the person(s) who can edit it, set limits on usable styles, word limit (etc) for each region, then protect the document to lock down the rest. (There is an existing Protect Document feature which allows this to a limited extent, but it doesn't fully support what is being suggested here).
You put the file on OneDrive, then share with & notify all the people who are required to contribute their individual edits. When they open the document, they step through the parts they can personally edit (as is currently done with protected documents) and make their changes.
Ideally, the definition of an editable regions can be driven programmatically, for example using ScriptLab's Word API. The API would have a TemplateRegion class which could be declared over a section of a document. The TemplateRegion's API would include methods for setting user access (who can edit the region), style limitations, word limits, etc on a region by region basis.
Once all the contributions are in (the single, shared document), you disable all editable regions and finalize the document.