I'd also restrict the allowed CSS to the tiny portion of the spec that styles the text and maybe some basic layout. Definitely no scripting allowed.

And require that if any colours are set, both foreground and background are set. (I've seen too much breakage with assumptions about one or the other.)

I still hope to see text/markdown as an officially supported content type by email clients.

Not useful proposal. Markdown is both limited and ambiguous. It's okay for writing by hand more naturally than HTML, but the sender should parse it and then explicitly indicate which characters are bold, which ones are underlined, etc, and you're back at a subset of HTML.

I guess it could be added to something like Thunderbird. Would be really nice.