I, too, would like to see this, but it'll be a lot of work. It's just not a top priority right now for me, unfortunately.
This is something I could prioritize if a business wanted to sponsor this; but right now the most pressing things are updating the docs, revamping our test suite, and I have a few action items for existing sponsors I need to prioritize too.
Anyway, no timeline -- but definitely something wanted!
I know I can write it in JSON, but IMO the greatest appeal of Caddy is its simple and easy-to-read Caddyfile!