Themes should be a collection of template/css/whatever files that can be unzipped to the right location and used by Hugo. That would need standards for the file types and directory structure, config settings for which themes to use, or which parts of the theme to use.

I was thinking about a WordPress replacement in Golang. WordPress themes can be zipped and shared as a bundle, which has made a large variety of themes freely available.

This is already the case. The source directory has this structure. /layouts is all the templates, /static is all the css & javascript. Just don't include a content directory and it would work just like a wordpress theme.