Field definitions would be another entry in the ACF admin menu where you could define any number of named fields with their parameters. The interface would be very like the normal field groups interface (though it would be useful to have some visual differentiation).
Then, when creating a field in a field-group, beside the ‘+Add Field’ button you would have ‘+Add Defined Field’, allowing you to choose one of your definitions.
This would be really useful for things like addresses that you might want to include in many field groups, especially for making sure that your naming conventions are not varying from group to group.
