I might get burned at the fandom stake here, but I personally think Zuko x Katara x Aang being a thing when they are older would be very interesting and hilarious. Just hear me out-

Katara and Aang, married, both looking at Zuko: “Hey so my wife and I saw you across the bar, and we saved the world together and all so, can I get you some tea?” kinda vibe.

Like the three of them are all each other’s narrative foils and parallels. They circulate each other narratively and emotionally throughout the whole series and even comics. When they’re all older restoring peace around the world, they would have all spent so much time together I like to think it would just, kinda Click for them