Google’s @-menu and Smart canvas applications have this feature.
By typing @email into docs.google.com (or selecting “Email draught” under “Meeting notes”), a template containing To, Cc, Bcc, and Subject lines will be loaded. Insert > Building Blocks > Email drafts are another way to get to it.
where everything (subject, to, cc, bcc, and body) will be “automatically generated depending on the email drafted in the document.”