How about implementing some sort of friends list where a forum user can add other forums members to it so that they could see which of their friends (or regular correspondents) are active on the forum?
Also, what about being able to view a list of people who a user recently sent messages to? Example, someone messaged me about an item I had for sale, I replied and deleted their message and now can't recall who sent me the message. If I could view a list of recent contacts that'd be helpful.
Unless there already is a feature for these (I'm aware of storing sent messages).