Description - When a recipient clicks on an expired tracked link, the resulting page is a 404 Error page. This is not a good user experience. It would be "nice to have" a branded page with a useful error message.
Why is this feature important to you - To provide the recipient with a not-so-ugly user experience.
How would you like the feature to work - Each external account could have a default expired link URL parameter. The Adobe user would populate the value with a nice looking landing page URL. When the recipient clicks on the expired tracked link, they would be taken to this page.