Similar to how collection fields are available in the HTML Embed code editor, allow Symbol Override Fields to be inserted as well.
For example if I go to a Collection page I can open up the HTML Embed Code Editor on an HTML Embed element. In the upper right hand corner of that window there is an 'Add Field' option.
This same option should be available when editing an HTML Embed element inside of a Symbol which has Override Fields.
I have a variety of landing pages I would like to have custom versions of similar HTML Embeds but it currently requires me to edit the code every time. If instead I could just use a Symbol with override fields that would save me a lot of work.