You are already using Part 2 (RELAX NG).

Part 3 (Schematron) gives you the ability to add highly flexible business rules to your schemas.

Part 4 (Selection of Validation Candidates) lets you write and reuse schemas written in any language and combine them to validate composite documents.

Part 5 (Datatypes) should provide a better alternative to W3C XML Schema datatypes.

Part 6 (Path-Based Integrity Constraints) lets you specify integrity constraints between elements and attributes.

Part 7 (Character Repertoire Validation) will let you specify which characters may be used in your documents.

Part 8 (Declarative Document Architectures) lets you make explicit information that was previously only implicit to your documents before validation.

Part 9 (Namespace- and Datatype-Aware DTDs) lets you upgrade and reuse your DTDs in the context of newer applications.

Part 10 (Validation Management) lets you combine these parts and plug in other transformation and validation tools.