I want more Dhall. I want types for data structures and small functions for dealing with repetition.

JSON5 is pretty nice, I hope it becomes the de facto standard at some point.

JSON5, and optionally it has trailing commas! It's all I use now.

More on JSON5 is here:

https://json5.org/

I like the comments and trailing commas too. It has a few other small improvements as well.

That does address pretty much most of my issues with it.

It's actually good enough to even write configs in it. I do hope it will get enough traction to be standard...

Curious why YAML doesn't work for you?

JSON is natively supported by many languages.

I don't know of any language that supports YAML without a 3rd party library.

Hmm, for sure, personally I find it a very small price to pay for everything YAML offers, but I understand this is a matter of taste and judgement where reasonable people disagree, and that there are problem spaces/organizations where adopting dependencies is difficult/has a lot of red tape.

Is it merely about the ecosystem? I'm curious if something about the language isn't meeting people's needs.

YAML is much too complex. I am never able to remember all the different ways of formatting text, for instance, and have to resort to cheat-sheet guides such as this all the time.

https://yaml-multiline.info/

The YAML spec is thousands of lines long (23k words, to be more exact). This is even longer than XML (20k words). JSON's spec is less than two thousand words. For a language that's supposed to be human-readable, YAML is maddeningly complex.

https://yaml.org/spec/1.2.2/

Plus, of course, you can execute arbitrary code with it. By design. AND it contains semantic whitespace, which is personally my least favourite innovation ever.

Then you'll want to look into JSONC.