Generally, Hugo isn’t prescriptive.
There is a limit to how much policing a small team can do, on something like community-submitted themes. Part of the reason there is such a wealth of themes to pick from, is precisely that there are very few rules about how you can submit a theme. Additional big differences naturally come from the fact that Hugo is not prescribing how you build your sites; rather it just assembles your pieces. That’s the way it should be, because I can use whatever library I want, or roll my own css and js as some do. I can call my shortcodes and partials whatever name I want.
DB-backed dynamic CMSs like Wordpress, Drupal, Typo3 etc, while they obviously have their uses, end up being quite hard to customize unless you do it with in the prescribed methods that come from whatever theme/child theme you’re using, or, within whatever plugins/extensions you’re using. When I need a db, I use a db that lets me embed its views and reports in web pages, then style them. Or in some cases, grab data via json, and import that using Hugo methods. When I have to touch a WP or Drupal site, I can feel a rash coming on, having to figure out what PHP pages I have to tweak, and not being able to easily save it to a git repo!
Many Hugo themes are simply that author wanting to scratch their own itch and design for themselves as a learning exercise. A handful have an obvious giant time investment to make them flexible for the many who might use them. It would be next to impossible to force everyone who wants to submit a theme to agree on a convention for the things you mention, like using the exact same shortcode name or image referencing method. Even more because of the many ways you can choose to do the same thing using Hugo.
It’s pretty trivial to search and replace “image: /path/to/myimage.jpg” with “thumbnail: /path/to/myimage.jpg”, and just “the way it is” in this field, in my opinion. You always have to look at your content and tweak it. Wordpress has the same problem with its “shortcodes” system, when changing plugins and themes in some cases.
But the idea of having a few themes that have the same constraints is a good idea, and probably there are people out there who would welcome that same-ness.
Why don’t you build some that realize your vision?