It would be great if the administrator could have more control over what the product owners can and cannot do. If product owners are allowed to create their own fields...filtering these fields in the list view can be overwhelming and hard to use. My suggestion would be to allow at a minimum the administrator the ability to restrict product owners from creating custom fields.
As you know, we have different levels of permissions for each user type. Product owners and contributors are able to create custom fields, while reviewers and viewers are not.
We have spent a lot of time in this area of users and permissions and do not have plans to make changes in this area in the near term. While we don't have plans to enforce this in the application, this may be something to enforce offline. I hope you can understand.