First of all, I just want to say that I love this interface. I haven't been as excited about an open source webmail effort in a very long time. I'm testing some extensions and wanted to know if there was already work underway to address these efforts.
There are a few things that I would like to add to this as optional facilities that anyone wishing to deploy this in their environment could do through the main.inc.php settings.
First, I'd like to add RSS feed capabilities similar to what Thunderbird already offers.
Second, I'd like to add blog support that lets you use the webmail interface to post stuff to your blog.
Third, I'd like to see IM presence enabled to let users know if any of their contacts are online as they are reading their e-mail and give them a quick access to initiating an IM session directly from their e-mail.
=Alain