A user recently submitted this to me:

I was wondering if it's possible to have more email notifications added to your (already very good) module? I installed it, and it seems to be working okay, but I'd like to be able to have an admin email notification sent out when a new subscriber pays via PayPal, and have this email configurable per subscription option.

It might even be better if your module ties in with Drupal's 'Rules' module: http://drupal.org/project/rules

I think it's a great idea, and will implement more email notification options.