The Daughter of Balance: The Tale of Princess Yue (Book One: Disaster Strikes)
Zuko x Katara, Aang x Toph, and Sokka x Suki are all endgame. Zuko and Katara are Firelord and Firelady and have three children-Yue, Lazuli, and Sokka. Zuko and Katara leave the Fire Nation for a while, but by some unfortunate incident, they don’t return. Yue waits for them every day. But when she finds out something goes wrong, it’s up to the Bloodbending princess, her Waterbending cousin, and Earth- and Airbending best friends to find her mother and father before it’s too late.