I don't think the BRB sign is good for anything since it only appears when clicking menus, friends, etc.

An AFK bubble that would appear when you maybe havent moved your character nor clicked anything for a minute or something before an AFK bubble would appear over your head. You can also trigger the bubble by just typing /afk.
I think this would be nice for when you're in a world, so people won't be asking you questions while you're afk and you don't want to seem like you're ignoring them.