I've been filling in the gaps in xcontents.dtx, and have a working
package that more or less matches the documentation.  The main thing I
think from using the new packages it the error catching needs to be
greatly improved.

There is mention in xparse.dtx (I think) of putting help texts in
\DeclareDocumentCommand, is that likely to happen? Similarly for an
informal description of the arguments in \DeclareTemplateType.  Are
there any more thoughts on how that could be done?  It would be