To be honest, one thing I've been interested in is a totally markdown-only web. You leave everything the same, you just use a Markweb browser as the only thing and it only accesses text/markdown. Then I build Yet Another Protocol Bridge for my blog and no one visits it ever again. That sounds like fun.
Check out the gemini:// protocal. There are browsers, search engines, and the whole thing is basically markdown.
its a cool idea, but lacks content. Discoverability is kinda bad as well. All fixable problems. I think it could take off given some good content.
The trouble is, people keep extending Markdown to add HTML features. There's even Javascript embedded in Markdown.[1] You'd just create churn, not a fix.
[1] https://www.markdownlang.com/advanced/javascript.html