I'm working on a somewhat similar system called Heximal. It focuses on adding full-fledged templating to the <template> element, and full declarative component definitions to HTML, plus some built-in custom elements like <h-var>, <h-out>, <h-scope>, <h-include>, and <h-fetch> that make HTML more of a declarative programming language.
A big difference from HTMX is that it doesn't rely on magic attributes, but instead adds full data-binding with rich expressions for any attribute or text content, and control flow, to <template>. And component definitions include defining properties, attributes, styles, etc.
These things are being proposed for HTML, so Heximal is somewhat of a polyfill for HTML from the future. Or it's a bit like Tangle or Curvenote.