I wanted to like asdf but 1. it's slow, and 2. not available on Windows proper (non-WSL). Same with mise (https://github.com/jdx/mise) which builds on asdf.
I've used all of these tools in conjunction with my own usage habits, which is why I'm developing vfox from scratch rather than based on asdf. so the issue at hand for vfox is the need to rebuild the plugin ecology. There's no way to utilise off-the-shelf asdf plugins.
I just looked at your plugin-template. This sounds good actually. Writing asdf plugins can be quite nasty. I like the approach of adding a lua dsl for plugin generation, not sure I like the dsl itself.
Volta exists but alas, it's only for JavaScript.