One of the best Avatar the Last Airbender stories I've ever had the privileged of reading, Three Families is one of the best paced and amazing stories you might find in Fanfiction. The story is an Azula and Aang Centric, dealing with those two characters in a world in which Azula from the future goes back and alters the past, creating a world that is both impressive to read and greatly written. James Golen, the author who wrote this story, already has two other Avatar stories that have recommendations on this site. The story itself also has fun, well written OC's that are NOT Mary Sues, and are in fact some of the best OC's out there. Another thing this story does great is it's background history, diversity of the world, and funny moments that will make you chuckle.
Three Families has just finished Book Two and has close to 800,000 words, and by the time it's done it should have over 1 million. That being said, the length does not make the story suffer at all. The story is certainly well paced and stuff that happens in one part of the book will eventually pay off in the next. The pairing as you read it leans toward a possible Aang/Azula pairing and maybe Katara/Zuko. Another interesting character is Malu, who is kind of part of the shows canon, as she appears in a later trading card game. Malu being a female airbender does bring a nice additional relief that there is more than one airbender in this story, although the two hardly ever react.
Again, this story is possibly my favorite Avatar the Last Airbender fanfic, other than What Should Have Happened In ATLA. I completely recommend giving this story a read. It's really worth it.