Since Email is still in beta, lets try to collect ideas and feature requests about email in one thread.
Webmail apps are pretty popular, so I would suggest checking out some GPL apps which can be added to the "goo" rather then trying to implement many missing features from scratch (IMAP, signatures, etc). Has anyone looked at:
http://roundcube.net ? Does anyone know of any other PHP/MySQL GPL web applications?
On the other hand, the current email beta is a very good start; maybe it would be better to stick with what's already there?