Let players send letters to other users, even offline ones, and they would check these letters in their menu inbox.
It would be great if you could attach things to the letters, like handwritten drawings, screenshots, avatars, items or even maybe permits to allow other users to enter your own private worlds whenever they want? I imagine streamers would get a lot of fan mail.
This would be really handy if I were selling avatar commissions and advertised in-app, then any buyer could just send a letter to me and we could work it out. When I was done with the avatar I could just send it to them via a letter and have a cost-on-demand lock on it so they would have to pay it to get the avatar and I'll get my money.