These suggestions were made in a comment on T297888: OAuth v2 client request form ignores default grant type values, which is otherwise resolved. Opening a separate task so the suggestion is not lost when the other task is closed:
- fail the form validation when there are no grant types - accepting a dysfunctional consumer definition and putting it in the approvals queue is a waste of everyone's time.
- show the grant types in Special:OAuthListConsumers. Missing from there made debugging T297888: OAuth v2 client request form ignores default grant type values hard for non-admins.