Currently Koha pulls enhanced content cover images using only valid (020$a) ISBNs. It would be nice to optionally include cancelled ISBNs (020$z) in the pool to check for cover images as well. I'm not sure how complex it would be to have a hierarchy; i.e. use 020$a as the source first, and then 020$z if none are found. Even if this kind of distinction isn't possible, it would be enormously helpful to include the cancelled ISBNs (optionally, of course).