If a new element is proposed I think it should be trialed as a custom element. And of course, once something is decided, we should have a polyfill to set the correct heading level (via aria-level) for older browsers.
The problem is trying to get browsers and readers to give them appropriate treatment.
Great news, that's exactly what we have!
I think that's what Javascript is for...
I thought that's what XSLT was for (All I know about XSLT is that it's a fearsome thing, so maybe I'm wrong).