Users should be able to define their email templates in the same way like PDF templates are defined: within template files that use HTML, CSS and PHP. Implementation may not be that difficult as most of the parsing and so on is already present for the PDF files. Implementation into the email process is needed.
Also, standard email templates need to be defined, I would suggest using these templates: https://github.com/leemunroe/responsive-html-email-template
Notice that CSS should be handled and compiled the same way like other CSS assets used by InvoicePlane: Grunt and Sass.