To add to what Jordi said, I want to mention that we do have an OpenAPI metamodel implemented in Ecore which we use in an intermediate step between JSON/YAML and UML (we first generate an OpenAPI model instance then we transform it to a UML model). This metamodel could be used for instance to create a custom notation with Sirius or to perform transformations.
After submitting my comment I checked out your repo and found the metamodel!
Yes, that can be a good starting point for other tools, e.g. a custom notation using Sirius. Since I'm maintaining the PlantUML integration in Eclipse, it's also relevant to use the textual PlantUML notation as another backend of the transformation from the metamodel to class diagram, possibly going through an Ecore model.
We use PlantUML to render models created with our Slack Modeling Bot, but there we just do a direct transformation from the UML Ecore model to PlantUML syntax. But good to now your extension exists!
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