"Filters" for calendars, mostly useful for external ones, both imported from a file or url would be nice :

Have something similar to filters in mail, to be able to have flexibility, and do custom actions :
- Hide event matching name/description x
- Copying event containing word x into another calendar (for achive, since you most likely will delete a url calendar when you are done using it) .. Or the ability to copy the current state of the calendar, with only what you want to see from it into one of your calendars before deleting it ?

Most likely in RegEX but Sieve should be able to do it too, since it would be similar to mails (?)

In the end, maybe it doesn't have to be that and could be only a few basic features, or broken down into a filter that can do basic things and something manual for stuff that would be a real pain to implement and not have any real value ?