Certainly VOTable is "quick and easy", and that would seem to be an
advantage. And it is obviously not enough for complex datasets. One approach
to this complexity is to build a format that encompasses most of the
situations usually found. Another approach is to provide these simple,
"quick and easy" components plus the means to build structure with them, and
I believe this latter approach to be a good one.
This structure is built on object-oriented concepts like inheritance and abstract classes (this is the benefit of switching from DTD to XSchema). This structure can then incorporate and extend either simple objects like VOTable or complex objects like XDF. I see a collection of XML document definitions (aka classes) that can be extended through inheritance -- for example a VOTable that has XDF objects in the table cells. I see development next of Image and Spectrum documents (among others) which inherit from an N-dimensional Datacube object.
With this model, we are not standardizing not the structure of the data, but rather the way in which data structures are created and described. It is then the community that decides what are the most useful objects that should be in the toolbox.
Roy Williams