In several occasions i had to msg someone and then couldnt due to a player being offline.
Im sure everyone would find this usefull to implement some kind of email system where players could leave a msg to an offline player. like /msg [name] [msg] and then when that player comes online he can read it with /msg read [name] .
to make access to the names u can also implement a friend system where players can befriend each other.
I know u can msg players on the forum but i think it would make this easier. Plus non of this would affect vanilla gameplay style that u strive to.