The TDML schema doesn't seem to restrict what elements can be found inside a tdml:defineSchema element. As a result XML content assist doesn't help you with those.

Exactly 8 things can be inside it: dfdl:format, dfdl:defineVariable, dfdl:defineEscapeScheme and dfdl:defineFormat, plus xs:element, xs:complexType, xs:group, and xs:simpleType.

Putting a choice of those as the content model would improve the XML support in the IDE for TDML files.