I'd like to see a Town Square feature which would include a noticeboard, allowing the player to exchange messages with visiting friends. Much as I appreciate all the buffs I've received from visiting friends, I wish that I could leave them all a note, in one place, telling them where their buffs would be most helpful on that particular day. Likewise, I'd like to be able to see such requests when visiting friends' islands.
On a related point, the mail feature which notifies you which friend has visited and what they buffed seems to be only intermittently available. I really wish that it could be fixed and made permanent, because when it works, it's immensely helpful.