It would be great to have the ability to automatically create an email address that matches a new blog's subdomain and will forward to the email address used at signup. OR the ability to create an email address that matches a blog's subdomain as a Supporter Upgrade option.
Here is a Email Forwarder Creator script for cPanel.
There is a cPanel Operations script but that only allows administrators to create an email address, not forwarders and it may not work with the latest version of Wordpress.
It would be nice to be able to offer a directory listing that pulls the data from the WPMU network database.
Reversing the direction of creation would actually be ideal. Imagine the user filling out a form that will create the data that goes into the directory and at the same time is used to create a Wordpress based site on a subdomain with matching forwarding email address.