DocBook has done a lot of work that could be reused.
I understand that it is a big change and Docbook is targeted on Technical documentation.
So it may not be a good idea, but:
DocBook defines (a lot of) elements that could be reuse: for example <span class="name-docbook-element"></span>. Then there would be a translation to Docbook on the backend.
Then the backend would be able to resuse the (very good) XSLT templates to create PDF and others output (HTML in one big page, one page per chapter) ...
It could also be able to import/export to the docbook format.
DocBook has done a lot of work that could be reused. I understand that it is a big change and Docbook is targeted on Technical documentation. So it may not be a good idea, but:
DocBook defines (a lot of) elements that could be reuse: for example <span class="name-docbook-element"></span>. Then there would be a translation to Docbook on the backend.
Then the backend would be able to resuse the (very good) XSLT templates to create PDF and others output (HTML in one big page, one page per chapter) ...
It could also be able to import/export to the docbook format.