A custom field type date range would be very helpful. Similar to the date range where you today select start date and due date but as a simpler more flexible custom field.
Use case: We create a lot of custom fields for tasks where we today need to create two fields, start date and end date. This could be i.e. our clients events, marketing campaigns or other task types where this date don't have anything to do with i.e. our workload. We need to finish the ads for a marketing campaign before the actual campaign starts, so our due date is before the campaign start date. But our designers making the ad, the media buyers and planners needs to know when the actual campaign is and when it is running.
If you could connect this field to the calendar as well that would be very helpful in displaying this information in an orderly manner.
I'm sure such a field would have other use cases as well :)