use an existing custom date field as the expiration date
If a post already has a existing field that can be used fro expiry date, allow publishers to set that as the default field
Agreed. Currently looking into how to achieve this specific piece of functionality.
Alex R commented
Yes, I agree this would be very useful. We have a site where users submit posts from the forntend. These are specifically for events that may last 1 day or two weeks. They input the start date and the end date of that event. So to be able to set expiration at one day after the end date field would be perfect.