It is (normally) not the whole document that is predetermined but I
want the possibility to predetermine large parts of a document.
Furthermore, I want to use the same mechanism (i.e. W3C XML Schema) to
validate. In addition to validation I want to use this schema (or
better call it template) to generate a valid XML file without further
information.
I use a modified version of W3C Schema now, i have an element called
"template" where I can predetermine fixed parts and also include
dynamic elements again. Looks like that:
...
<xs:complexType name="colorsType">
  <xc:template>
    <color>red</color>
    <color>blue</color>
    <color xs:type="colorType" xs:minOccurs="0" xs:maxOccurs="4" />
  </xc:template>
</xs:complexType>
...

This is ok for my purpose and it works as an "XML constructor" as I
provide initial values for regular pattern values for example. But I
have to write my own validator which is quite a lot of work. You
schematron suggestion is nice for validation but doesn't work for
creation. I need it all in one file.