What about an option to leave a note on someone's house? I might not have someone on my friend list, but might want to leave a message to meet somewhere, bring something, etc. I was thinking something similar to a friend chat as far as length, and maybe a low limit for how many messages one character can leave for another (maybe allow overwriting of last message or something).
There are some abuse/harassment concerns, and unlike friend messages that exist for a short time this would be something that stays there for awhile.
I know, I know... coming home to all the junk mail, just like real houses!

Old soldiers never die, they just fade away.