How about letting paid members be able to track messages - that is see when another person has read a message.

Since I don't have the time to keep track of a user and what a user is doing - but I sent them a message but I don't know if they have actually read it yet, or if they are one of the people who does not read their message box very often. If there was a way to get a "Person A read your message at XX:XX:XX time" would be a cool way to keep track of people!