Although it's probably impossible to have an XSD schema that has every possible configuration element,
it's possible to provide a valid XSD that allows any element at certain places, but does have a list of the common (= minimal set) of configuration elements, such as <configuration>, <appender> and <root>.

How to have a specific element that can have any child element?

<!-- The element  "appender" can have any child element, for example "foo" or "bar" or "myOwnElement" -->
      <xs:element name="appender" minOccurs="0">
        <xs:annotation>
          <xs:documentation source="version">1.0.0</xs:documentation>
          <xs:documentation source="description">TODO The appender ...</xs:documentation>
        </xs:annotation>
        <xs:complexType>
          <xs:sequence>
            <xs:any minOccurs="0" maxOccurs="unbounded" processContents="skip"/>
          </xs:sequence>
        </xs:complexType>
      </xs:element>

Benefits:

• Can be used to validate the xml in IntelliJ/Eclipse, without having to run it.
• Code completion for XML in IntelliJ/Eclipse
• The XSD description can explain what those "minimal" elements do. With IntelliJ (and Eclipse?) that "javadoc" pops up when writing it.
• Can also be used by logback itself to validate the xml at runtime.