One idea that I haven't seen brought up is a banking system in the game. It could serve as a way to save larger amount of items, much like the closets in a house do now. You could have players buy a box at the bank and charge different amounts depending on how much the boxes hold. Also, you could deposit money and get a daily return, maybe 5-10%, based on how much you deposit.
Not only would the banks be functional and useful, but there are ways they could tie into skills and every aspect of gameplay and character interaction.