I recently upgraded to 4.0.3 to resolve an issue with reocurring events. I had planned on allowing unregistered users to 'see' the calendar and the events, but be prohibited to view the details without the appropriate permissions. So I set Can View Calendar to yes, unregistered users could see the calendar but no events, which I found strange. Why show a calendar with no events I propose to allows users, where permissioned, to see calendars with titles and links to events, but when an event is clicked, then check permissions to View. That way they see the event title but no detail, unless the have the appropriate permission(s).
So my suggestion is a feature rich calendar with the following:
Add ability for users to view a calendar with it's event titles, but require additional permission to see the detail of the event
Full suite of widgets including upcoming events with admin settable format, choice of x days
Add option to add link to Add Event in all Calendar widgets.
Allow users to click and add themselves to an events list. This way you can see a list of attendees.
Allow events to have a maximum number of attendees. Allow users to delete themselves from events.
Allow events to have a Paypal or other pay link so you can have events that require payment.
Create a "Events I'm attending" (see above) widget
Create a mini calendar widget that links to events in the Calendar notitems that have published bia the CMS
Allow events to have attachments
I'm sure there are others out there with other requests. So lets hear them!