Add a new option to the Filters settings to play a unique/custom sound per filter, other than the defaults. This would allow uses to hear a notification that would be important to them other than the generic new mail alert.
Custom Sounds can then be used to:
1. Alert of Important or High Priority emails
2. Spam that your are checking or keeping an eye on
3. New Facebook Notification
4. Your Pay Slip has arrived
5. Appointments from the Doctor, Dentist, etc
6. Whatever you need and so on