The first part of this functionality is there already.  When you create a Notification Forum simply check "Notifications" for whatever role you require and both new threads and reply notification emails are automatically sent.

However, allowing members to remove themselves from this notification is not really what the intention of the mechanism is about.  This is to ensure a specifc group of people *always* get those notifications - like moderators, beta-testers, tech support teams, etc.

Cheers!

As someone who manages several community sites (clients), I have found that most people get pretty upset about emails even if they have the option of turning them off. Lots of emails to admin or support will say something like "STOP SENDING ME EMAILS FROM FORUMS", and that is verbatim. Because of this, I worry about adding features that are going to do users more harm than good.