Wow, I had the very same idea. But everyone I told the idea basically said: "email is a horrible protocol stack, you don't want to build anything on top of it" or just something like "email is dead". I still think it is good idea.
email is a fan-fucking-tastic protocol stack BTW. It's detractors all want to do something the user doesn't want or need.
Ping me at sethjgore@gmail.com if you agree. I'm organizing something and I want to have people who think email is still underused onboard.
Ping me at sethjgore@gmail.com if interested.