In a custom component, I should be able to define my own skin parts and not just states. This will make my component more resuable. For example, I might want to have an icon field in a custom button component. If I can make that icon field a skin part, then I would set the value of the skin part from the properties panel, just like label is set in this way. Limit the different types of skin parts that can be created but get the feature in.