text/email Mozilla/5.0 (KHTML, like Gecko)
Eventually, if everybody switched to sending text/html-for-real first, maybe the email clients could follow along and make text/html also mean "the newest and most featureful version of HTML5"; and then we could all revert to just calling it text/html.
Rather than a distinct MIME type, I believe it could also make sense to borrow the HTTP header X-Content-Type-Options: nosniff (https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-...) into the MIME envelope of the text/html document, to signal that this envelope is "really" using HTML, no second-guessing. (This is how the header was used in IE8: to tell the renderer to explicitly not trigger IE historical-renderer compat mode.)
That might get into a situation where the message has two MIME bodies that are both text/html, though, and I'm not sure existing clients have been written to cope with that. A well-engineered solution would work with existing clients (by having them ignore the body of unknown-to-them media-type, and render the old fallback body of known media-type.)
For a while, people would be sending both text/html and text/html-for-real, but eventually we’d maybe be able to switch to only sending text/html-for-real (while also continuing to send text/plain for fallback.)