Have the ability to save the message you are composing in a folder other than the default Sent mail.
I am coming over from using roundcube and horde, this was one of my favorite options. I do not wish to go back to using them because one of the reasons why I like Virtualmin/Webmin and Usermin is because you have so much more advanced control inside Usermin.
I love that it is already setup that I can download and upload files inside Usermin and have the ability to attach and detach a file on to my user account.
So, that's only thing left on my wish list for usermin (because I had already read there wont be a calendar because of php).