Description
Supporting more advanced customization via template syntax for docx would address many pain points when producing docx format with Pandoc. This would also be a general solution for issues such as #3109.
Microsoft Word supports special content controls that allow marking parts of the document with special tags. This allows for robust XML copy/pasting between the template and the output document in pretty much the same way as with other formats that currently support templating (and interpolated variables).
Content controls also allow for automatic metadata insertion, which is already partially supported in certain cases with Pandoc - e.g. setting certain set of metadata tags and adding a content control to the head of the document allows for special markings to be defined via markdown, such as "Confidential" in the header or "Date" in the footer.
Combining what is already supported with custom segments via content controls would allow for a lot greater flexibility in Pandoc template design (e.g. not forcing metadata output by default as is currently implemented).