Enhancers seem to be a great concept. Still there aren't many of them by default, but it might be super helpful to have them for convenience for the Drupal core entity types typically referenced, e.g.
What I'd expect from these enhancers is to provide typical results, like the values as array / json or the image URL or the term id / label, etc.
Also see related issues about enhancer examples and documentation so that community could help easier.
Implement enhancers for references that allow to trim down these references to the first level.
e.g. Node > Media Image > File Entity > Image to Node > Image so that the data from the image can be accessed as if the node directly had that image.
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3.0
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+1k to have enhancers for entity_reference AND entity_revision_reference fields (👀paragraphs...).
Thanks for the feedback @matthieuscarset. The (feeling dirty) workaround for now is to use https://www.drupal.org/project/jsonapi_include → to get at least a bit more flattened structure. But it would be far better to solve this in jsonapi_extras using the field enhancers as described.