Allow sections to appear at the same level as chapters. That is, allow a book to contain chapters or sections.
This would allow "traditional" DocBook users who want and need to use chapters to continue using them, while allowing other users the flexibility to work with one less level in the hierarchy.
The DocBook Technical Committee discussed this request. The Committee is developing new topic and assembly elements to better support modular documentation. A topic is a more or less standalone unit of content that can be assembled into large documents. In the latest draft, a topic could appear as a child of book as well as chapter. That would seem to satisfy the need for greater flexibility in modular content.