Markdown would be a pretty good fit as a compromise between HTML and plaintext for email. I wonder why no one has thought about that yet.
Markdown syntax takes a bit of time to learn, but with images, tables, limited HTML tags, I think it would suffice for 99% of use cases and remove the need for all the sanitization that current clients do.
Markdown is an authoring format. It’s OK as that. But it’s completely unsuitable as a publishing format, because there’s way too much variation of interpretation. Remember also that Markdown is built on top of HTML, more or less—you’d have to either ban all HTML tags (… which rather makes it not Markdown), or have just the same problems as before, but now worse.
People have suggested Markdown for email quite a few times. It’s a total non-starter, absolutely no chance of anyone doing it in the next few years at least (and I strongly doubt Markdown in any form will ever make it in like that).
Mail clients would need to learn to handle the markdown MIME type, and realize it could be plain text, or run through a converter to show as rich text.
But sadly too many folks use mail clients that render and compose HTML, or worse, and there would be no real incentive to switch. It might be a function of the sending client to send the prerendered HTML along with the plain text markdown part.
Markdown syntax takes a bit of time to learn, but with images, tables, limited HTML tags, I think it would suffice for 99% of use cases and remove the need for all the sanitization that current clients do.