We don’t know much about the Northern Water Tribe’s royalty or chiefdom, but what we do know is that Yue’s father was the chief during The Last Airbender. Then, in Legend of Korra, the chief of the Northern Water Tribe is now Korra’s father. Now, if the title of chief is passed down through families, it’s possible Yue’s parents had other children after her.
Yes, we literally saw the Avatar Cycle destroyed in season two of Legend of Korra. When Azula blocked Aang’s chi and blocked his connection to the other Avatars, he was able to meet with the four Avatars before him through a Spirit journey. So, theoretically, Korra could do a more intensive quest through the Spirit World to find the past Avatars and reconnect with them.
Lin and Suyin’s fathers were never outright revealed in the show, but looking at them side by side, there are distinct similarities that hint they are half-sisters. This specific theory states that since Suki died young, Sokka and Toph might have had a romantic relationship at one point, and he’s Suyin’s father. Their basis for this was Suyin, and her children resemble Sokka in looks, expressions, and personality.
First and foremost, she looks like the Air Nomads. She has grey eyes, and the only other person in The Last Airbender that had grey eyes is Aang. Her personality aligned with Air Nomad’s teachings of “violence last.” She never sought to hurt people and just did what she had to survive. The next reason is that she knew a lot about energies, auras, spirituality, and chi.
Because we don’t know how old Appa or Naga were in the shows, there’s a theory circulating that they’re the same age as the Avatars, and they reincarnated with them. Guru Pathik said the Avatars and their animal guides’ fates were intertwined.