Currently, editor for the Content Fragments, Metadata Schemas and Folder Metadata Schemas does not have a documented way to add custom components into the editor. This is incredibly restrictive when you want to do things which can be done in standard AEM Dialogs such as:
Drive Dropdowns off a list of values
Create Multi / Multi-Composite Fields
Customize Field behavior
If there was a documented way to extend / customize the fields, this would allow implementors to better meet their business needs.