On my Christmas holidays I built a little tool that does similar things.
Features I wanted to include originally were
1 shell completion autogeneration from `--help` texts;
2 support bash/zsh/fish;
3 support toml-like description of command arguments.
At the moment I have a tool that generates completions from `--help` messages and supports bash/zsh. I use it daily and it makes me little bit happier.
Actually when I started this project I used fish as my main shell on laptop and first versions of cod did support fish, but then I decided to focus bash/zsh as they look more popular.
I still want to add fish support when I have some time though. It doesn't look complicated.
There are some discussions about cross-shell autocompletion on the #shell-autocompletion channel of https://oilshell.zulipchat.com/, but it didn't go that far.
The Ion work described in the sibling blog post went the furthest in terms of implementation I think, but I haven't heard anything about it in awhile.
Syntax here is not anything like a well thought out proposal, just a demonstration of what I believe to be a shell agnostic method of describing both 'where can I complete?' and 'how can I compete it?'
I wouldn't want to limit myself like that. The best home spun completions I've written will do things like look up values in a database, differ depending on previous arguments, parse some file, etc. These will never fit into a declarative language.
Features I wanted to include originally were
At the moment I have a tool that generates completions from `--help` messages and supports bash/zsh. I use it daily and it makes me little bit happier.[1] https://github.com/dim-an/cod