Possibility to customize fallbacks more specifically and not only from other model fields. Maybe using methods for fallbacks or calling global settings. Making sure the customization is also shown in preview to prevent mismatch on generated meta data.
Use cases:

• Adding custom fields to site settings for default content, e.g. a site name, site description and image upload to be used if page meta fields are empty
• Customizing page meta title to include site name, e.g. "Page title | Site name"
• Using a static file as fallback for meta images