Now with Stage Theory most likely being true I want to bring back my Theory on who the potential third avatar character is
Azula - The third avatar stage appears to be Omashu, Omashu is where Aang and Azula first fought, This is before they even met Toph. Azula is a popular villain so that would be interesting
Zuko - The Western Air Temple is where Zuko joined Aangs team, So it would be funny if the Omashu stage was Aang's stage and The Western Air Temple was Zuko's stage Appa and Momo appear on the Air Temple Stage, this theory is BUSTED
Toph - This is just because Omashu is in the earth kingdom. But Toph did not go to Omashu in the show.