there is already an option for getting notifications both in forums and through email.
to subscribe to a thread, click on thread tools above the original post, then click Subscribe to Thread. there should be some options. if you want threads to send email notifications always, go to edit options in your user cp, go to the messaging & notification category, then default thread subscription mode. then change it to whatever you want. unfortunately, there is no way for subscriptions to reach through PMs and ingame.