I have two recommendations myself that I would love to see implemented at some point. 

One, I would like to add the opportunity to accept or deny friend requests instead of just simply ending up on someone's list (or at least get a notification) as this ruins the "Invisible" feature entirely. Secondly, I think notifying people when you respond to their visitor messages would be absolutely rad. Let me just say that this has caused quite a few miscommunications in the past.