There is a process: it’s called “opening an issue with a strawman API in GitLab”.
You need use cases in multiple applications, because we don’t add API just because somebody, somewhere, wants it for their own use case; GTK developers are a slowly renewable resource, and maintaining an ever-increasing API with restrictions on backward and forward compatibility requires being careful in what we add. This also means that a lot of times your requests will be denied unless you bring a strong case for them.