Now that the game has a functional notification system how about adding notes about sold (expired) items, off-line messages and even guild bulletin board.

All of those features have been asked for many times before, but there's never been a facility for it. Well, now there is.
The personal messages could of course be limited to friends-only and possibly only a few per toon. Same thing for guild board, heck, even one note per guild would be great for organising gettogethers and such.

(and this time I have to admit that I didn't even bother to search for priors, because I know there are but I'm not gonna check each of them to see if the SL notification system was mentioned)