You can already use the multipart/alternative MIME type for that (which is what HTML emails use), it's just a matter of email clients deciding that they'll show

You can already use the multipart/alternative MIME type for that (which is what HTML emails use), it's just a matter of email clients deciding that they'll show a Content-Type other than text/html if it exists. For example, you could use just a plain text part and a multipart/alternative with a text/markdown Content-Type. The email client would render Markdown if it's capable of handling it, and the plain text version if not.