Combine Rules and Filters capabilities
Rules allow message bodies in the criteria; filters allow performing actions other than simply blocking a message or adding to spam score.
Combine these so messages can be filtered, blocked, redirected, etc base on any criteria possible.
Also, it'd be nice to be able to add a block of text at the top of a message such as "WARNING: This message contains an attachment and the sender is not in your address book; proceed with caution"
I guess that also means you need a rule to check the recipients address book, huh? :)