Would raise my hand to volunteer for the reference implementation. I guess it would need to be in C++/Rust ? RFC, however, involves way too much talking and also needs solid networking amongst the web crowd. Not qualified for that. For a template language, it would be better to copy a subset from an existing de-facto standard like jinja2 which already has a lean, performant subset implementation at https://github.com/Keats/tera.

Document/template inclusion model should be OK now in modern era thanks to HTTP/3. Not really sure how that should ideally look like though.