I looked a few pages back and didn't see anyone requesting this so I thought I would. What would really complete roundcube for me is a calendar.
Where would you store it? The point of RoundCube is to be an interface between you (or any user) and an IMAP server. Currently I don't know of any IMAP server that includes a calendaring service. Not saying it's a bad idea, could add some nice features; however, in the grand scheme of things, I don't think a calendar is really a necessity right now. Perhaps later in the development process though.
Since it is probably a feature to be implemented down the line, it might be possible to interface with some other FOSS calendering system that would take the same username and password as the email. Something with iCal support maybe so that it can be subscribed to from RoundCube.
I personally use RoundCube as a substitute for Squirrelmail, which stored things like this on the server so it wouldn't matter to those who migrate to RoundCube if a calendar were stored server-side.