Too many ways to build compound documents

I think that are too many different ways to build compound documents, either
by reference or by inclusion.
At the moment we have:

1) XHTML2 embedding attributes
2) XHTML2 / 5 object
3) XHTML5 embed
4) XInclude
5) XLink type="embed" / type="resource"
6) CSS content: uri(); property
7) DTD external entities
8) XBL or XSLT (which could transform markup across languages)
9) Direct inclusion (ie plain writing or server-side processing)

Plus we have media specific, like svg image, xhtml 2/5 img, smil media
object, css background-image / border-image;

I was wondering if any work was started to provide a unform way, instead of
relying on single language capabilities (which seems the current orientation
of CDRF and CDIF)

Giovanni