I would love to hear more when you're ready. The major risk is that every spammer worth his salt will sign up and then use it to generate spam. As a result your domain would get listed in the RBL's and emails would get blocked every time you sent them... essentially devaluing your domain name.
There are checks and restrictions that can be put in, such as caps on the number of emails that can be sent out per hour for each user, with instant admin notifications when spam is suspected, so still definitely would be a great extension with high demand if done right IMO.