I'm waiting for the recursive add-on: Someone creates an add-on, hosts it on Heroku and then enables the add-on in the add-on app.
I'm already building an addon as a Heroku app. Of course, I work for Heroku, so it probably doesn't count.
Re: Heroku: Announcing the Add-on Provider Program
#11You're missing an important detail: Start using the add-on in the Heroku app. Since the add-on provides feature x, you can simply depend on it and remove feature x from the Heroku app :-)