I am coming here because I own the Yahoo Mail Group which gathers about 5,500 people worldwide. Most of them are pro-users and usually chosen to test the latest Mail beta features. The group is followed by the Mail Team who let us know the latest improvements and discuss bugs.
Over there you'll also find great ideas, especially concerning third party applications. The only problem for us is that we are not developers. Therefore a moderator told me that it'd be great to sort of let you know of our presence. For you this maybe a place for inspiration by reading what users are expecting. This may also be a place where you can get a pool of users testing out your applications and publishing feedbacks for you.
If you are interested in getting to know more what enthusiast users are wishing for, keep this URL in mind :