As everytime this came up before - Media Sub Type will never be fully customizable because its values are used internally to control playback behavior.
Its also important to remember that its not meant to be a replacement for "Genre".
Media Sub Type should clearly identify the type of content, ie. Movie, TV Show, Music Video, etc.
A "Kids" show for example, is still a TV Show or a Movie, its not a new type, its just a specific categorization either fit for "Genre" or maybe "Keywords"