I would like to be able to have an offline messaging system for friends to talk while not in the game. This has been mentioned before, I just thought I'd bring it back up as it's important, in my opinion!
Simple profile commenting would be a great start, but an actual public/private messaging system would be ideal.
Also, slightly related: maybe a way to invite people into groups, like as a suggestion between friends. Not as big of a deal as the messaging point, though.