Certain Wikibase API modules, e.g. SetClaim or SetReference, do have a field that must contain a JSON blob. Even if these API modules are not made to be used manually by a user, we want users to play around with the API and learn how it works. That's one of the reasons why we have the ApiSandbox.
Problem: The fields to input JSON blobs are tiny single-line, 225px wide (see ApiSandbox/resources/styles.css) <input> fields. Editing a JSON blob in this field is pretty much impossible.
Suggestion: Add support for <textarea> fields to the ApiSandbox, so extensions like Wikibase can make use of it when appropriate. Set a low default width and height (e.g. 3 lines height and keep the 225px width) and set resize: both;.