Why ? As has been revealed in several discussions before, it would be very limiting for DTML to force compliance to XML (dtml tags inside html tags, etc)
Making it only superficially similar would just confuse people.
If you consider separate code and data. If you think about putting XMLDocument in real use. This is one of the possibility: using an xmleditor, the user can save the xmldocument to zope server as zope automatically make all parts of the document reusable as zope object (is it all what zope about? making a document into many small parts as each of them be re-usable object?) This is process by zope automatically, the author do not required to do extra works. The author can edit the document or part of it and view the final result if the xml-editor support this. To make this better, the author can embed dtml code in the xml document, which make the document be active and live. You may have a system automatically updated. Isnt it what zope designed to be? The document inside zope can have multiple view as we can have dtml-methods to transfer them into different view: html, xml+xsl+css, pdf, doc, etc... If dtml syntax is not xml-complience, all of these will not happen. A deeper change is to make dtml syntax more xsl like. I.e., make it block oriented. So, the author is forced to code in good manner, instead of the opening and closing tags were spearated into many places which I think is very bad for maintenance.