I should be able to generate a subject line from a CMS collection to give an indication of information an end user wants based on the email link they click.

i.e. A blog post about "house cats" with an email link should be able to generate an email with the subject "Tell me more about your cat!"

Your curent URL linking even filters out "mailto:someone@yoursite.com?subject=Mail from my Webflow Site" as an invalid email address. This is basic functionality.