In one of the episodes in book 3 of the legend of korra they mentioned that Toph left to travel around the world in search of enlightenment according to Aiwei nobody has seen her since. In my opinion, I really hope they talk about her more in book 4 or at least show her. I mean I think they will since they mentioned her.
Also I think they should talk about Azula and what happened to her. I'm not a big Azula fan but I think she deserves to be mentioned and shown too. I hear there is rumors that she is the old lady that helped korra gain her memory back but then again I don't really think so because the old lady looks more like Mei. Anywho I can't wait for book 4 and hope is as good as book 3