I'd like to see an option to ignore certain tags when determining if a track is "perfect". This is different from "preserve tags" because Picard would still do whatever it's going to do with the tag, it just wouldn't take changes to that tag into account when determining whether a match counts as perfect.
For example, MusicBee automatically writes a Display Composer tag when there are multiple values in the Composer field. I have "clear existing tags set" and Picard always removes Display Composer, which is fine - good, even, because if Composer changes then Display Composer needs to be updated, too. But if Composer hasn't changed, Picard doesn't see the track as a perfect match because it's still removing Display Composer. If this were implemented, Picard would remove Display Composer if the file were saved, but wouldn't show it as a change when deciding if the match is perfect.