Use of other XML grammars/schema/namespaces (needs terminology correction/specificity)

In using XML as a syntax language, we can take advantage of the work done in other disciplines that have standardized implementations of elements used in genealogical data. Listed are some candidates for use.

RELAX NG or XML Schema?

Which schema language (i.e., language to define an XML-based data model) should be used?
Here's a very short article on the subject. Its verdict: RELAX NG

RDF

Resource Description Framework is an older XML framework for the description of elements. This is probably mostly useful for understanding how to use XML in context of this project.

Geography Markup Language (GML) Family

GML is an XML language that describes geographic places in predefined ways. This is one possibility from which a schema for the description of place names could come.

TEI P5
TEI P5 is a very diverse schema that was originally designed for marking up scholarly papers online but has many other practical uses.

XML schemas for Time/Date/Calendar

Schematron is another ISO/IEC Standard markup which is great for standardizing things such as date-time formats (as well as social-security numbers, telephone numbers, zip-codes, etc.). The great thing about Schematron is that its rules can be embedded as a layer on top of W3C XML Schema.

XML schemas for sources/evidence, citations

METS/EAD ? (Call for Comments)

The idea of templates: Use of other XML schema (see below); User-defined templates for surety, naming conventions, place names,etc.