I would like to suggest that card templates and the styling info be stored as .html and .css in a folder in the user profile directory that is accessible from the file system.
Reasons why this is a good idea:
- Making complex card types with lots of javascript will be much easier
- Easy to access and backup
- Easy editing with external editors, without needing add-ons or AnkiConnect
- Version control with git
Maybe something else that would need to be added is a “refresh” button in the card template editor, for if you have made external changes to the files and want to see them immediately in the card template editor preview pane. This would make debugging a little easier (especially when combined with the add-on that gives access to Chrome developer tools in Anki)