Enhancement request: Improvement to custom fields of type "User"

Hi,

Now that we have the ability to create and add people to Teams, it would be great if the "user" type in custom fields could also allow the user to be restricted to a selected team.

For example, on Requirements, I have a custom fields called "Test Manager Assigned" and "Project Manager Assigned", but it's possible to select any user; restricting it to the "Test Management" and " Project Management" teams respectively, would be vey useful.