CADL from Microsoft: https://microsoft.github.io/cadl/language-basics/overview
Smithy from Amazon: https://smithy.io/2.0/index.html
JSON Schema IDL vocab (not a thing yet, and doesn't sound promising at all): https://github.com/json-schema-org/vocab-idl
And I know there are many IDL, but I only pick the ones based on their API related work.
OpenAPI's JSON Schema is definitely not designed for data modeling that's why the generator is a mess.
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I'm personally inclined to Smithy which seems to be the most battle-tested with real and large use case. And there's a better chance to convince my boss "it's from AWS", and generating OpenAPI file from it is also aligned with "standard" vibe.