I would greatly appreciate something that interacts with the calendar to help schedule events. Particularly, a way that the GM could create a list of possible times, have players choose which work for them, and allow the GM to select the option that works best for everyone (much like a Doodle Pool @ doodle.com). Also, a way to have players confirm events on the calendar.
As for how these features would look: For the availability polling, I think just a box below the calendar to create availability polls would be great. It could sorta be a subfeature of the calendar. As far as event confirmations, there could be a way to click on an event on the calendar and click accept -- maybe on a new page.