It will be nice to have:

1. DocumentType.getCustom(String key), DocumentType.setCustom(String key, String value), Property.getCustom(String key), Property.setCustom(String key, String value) to be able to save additional metainformation in the schema.
2. linkedType (useful for List, Map and etc.) and referencedType (for embedded, Link, Lists/Maps with links) on Property
3. Type.CUSTOM, Type.ANY - or at least Type.SERIALIZED
4. Some validation/constraints on property values. For example: not null, min/max, pattern and etc.
5. Property.setType(Type newPropertyType). Type of a property currently unmodifiable.
6. Ability to specify default value for a property
7. Document (or DetachedDocument) implements java.io.Serializable. Or at least RID detached from database.
8. Separate interface to mark objects which are mutable