I'm only wondering what will happen with characters from previous books. I don't remember many of them in Legend of Korra series when I watched them and it would kinda feel like a waste if they all were to just disappear...
Also I wouldn't really want to bang "granny" versions of them either
I'm not saying it wouldn't be nice to see old characters in a new book but wouldn't it be weird to meet them as old versions?..
That being said anyone could remind what happens with characters like Ty Lee, Azula, Suki and others during Korra times in a series?