I suggest being able to send gold to someone with mail. I would want this because I know a lot of people want to trade gold for skins, art, signatures, etc but aren't able to find a time with the seller because they live in a different time zone. One way I could see this being implemented is in the middle of the mailbox GUI you can click a gold and it tells you to type the amount you want to send in chat, similar to how you trade gold.