End users can share a calendar with anyone from Exchange using iCal or HTML.
Extend this feature to allow authenticated users (either via URL, or username/password) to edit calendars shared via iCal, updating the same underlying feature to use HTTPS rather than HTTP.
This will allow end users to collaborate better with anyone, whatever the platform, or whatever client they are using. This will also mean that users of mailboxes who wish to use the client of their choice will not be second class citizens.
I am aware that the underlying protocol is CalDAV and the PG may not want to reintroduce WebDAV, however I think this exception is worthwhile.