Great idea. My suggestion: why not use the Gmail-style johndoe+spotify@ suffix? Just because people would be more used to it. That way johndoe@ also would work.
An irritatingly large portion of websites don't let you put + in email addresses.
I ran into an issue where using the + notation required me to create a whole new account on airbnb because I had forgotten that I used + in my original email.
Thanks thewarpaint. Good point. Having read the below counterpoints though, I'm not quite sure now! I'll look into it.
On second thought they have a point, I have never had an email address with a dot being rejected, but I've seen it for the plus alias several times.