I get what you mean, but it's not like we'd have to keep adding these on. "text/html" on email clients
currently means "some specific ossified version of HTML4"; but text/html-for-real would hopefully mean "the newest and most featureful version of HTML that your rendering engine can manage." So there'd just be the one version of it.
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.)