Custom fields of User format should allow limiting down the list of available users by either Role or Group
Doing this would allow limiting values that can be chosen for custom fields to only users who are in a specific department or who have a specific defined responsibility. The goal here would be to define a role such as Testers and a corresponding field called Designated Tester that would only allow selecting those with the Testers role.
It sound simple but there are likely going to be questions about whether or how to handle cases where users change roles after they have been assigned to such a custom field. In my particular case I'm not concerned with these cases too much since we don't anticipate frequent changes in roles and if the role limitation were only to affect the ability to assign new values to the field it would be a big improvement over the current behavior in which all users are shown to pick from.