Add Advanced Feature to custom user fields that allows you to only show a subset of users

We'd like the ability to narrow down user fields to a specific subset of active users. For example, we have a custom field used on a couple different record types that includes a list of project managers. We'd prefer to use a user field for this purpose but since we are unable to limit the available options for end users to select, we had to use a different field type.

I think this could easily be accomplished if you created "Tagging" as the user level. For example, be able to go to a specific user account and add a tag to them and then at the custom user field level, you could have additional options to show all users vs show users with tag X Y or Z.