Identify places where we could dogfood our RESTAPI

Outside-In: impossible in its current form/direction, because it heavily relies on forms. We'd have to automatically generate forms on the client-side, and port much of form API to JS.

Quickedit?

This we can do at any time, but as long as most fields use the \Drupal\quickedit\Plugin\InPlaceEditor\FormEditor in-place editor, that may not actually be A) feasible, B) gaining us much.

Inside out?

Answered by #3.

Tests?

Arguably, we've now done that, in #2737719: EntityResource: Provide comprehensive test coverage: for every entity type, every format, every method. In there, I've got extensive test scenarios from a front-end developer/API consumer POV. Including all of the mistakes they might make, and ensuring that the useful error responses are sent.

…?

Thanks to #2737719: EntityResource: Provide comprehensive test coverage: for every entity type, every format, every method, if a future opportunity arises for a new feature to be built on top of REST, we can then actually be confident that it's possible, because that's already doing a ton of dogfooding.

Since there's nothing else that's actionable here, closing this.