I've looked at the codebase and there isn't much I feel would do well as an extension. As I said upthread, most of the code is AJAX, input gathering, history support and then book-keeping/event stuff around that.

There will be a bunch of htmx-like libraries coming out though and I expect many of them to take different design perspectives. Two that I'm aware of are https://data-star.dev/ and https://ajaxial.unmodernweb.com/, both were created by people on the htmx discord. I know of at least two other minimalist rewrites htmx discord users have done, but they haven't published their stuff yet.

If you'd like to create your own, I do an overview of how htmx works here:

https://www.youtube.com/watch?v=javGxN-h9VQ

I think a minimal, clean room, fetch() based implementation could be done in a weekend.