In the Legend of Korra it was hinted (and also written in the wiki) that Druk (Zuko's dragon) is a dragon descending from the ancient firebending masters Ran and Shaw. After rewatching the episode The Firebending Masters (ATLA 3x13) I noticed that when Zuko picked up the Sun Stone (that the Sun Warriors cared so much about) he said that "It feels almost alive" this means that its a dragon egg, and not just any dragon egg but probably the egg that Druk will hatch from! This is why its the sun warriors most valuble treasure!