It would be useful to have the owner's name, group name, and permissions added to a package's CONTENTS file. This information could be used to ensure package integrity -- much like tripwire or, from what I've heard, linux-audit. The package manager is in the perfect place to record this information, half the information is already in place (checksums used for uninstalls). My personal use for it would be for changing uids and gids en-masse (as when setting up an old install to use ldap et al).
Steps to Reproduce:
Unfortunately the current CONTENTS format isn't extensible at all (existing code assumes a fixed set of fields and would break if the format changes).
There are plans to change the installed package database format itself though, including a more efficient and much more flexible way to track installed files where this information would be added (it's useful for package upgrades as well).