One-shot X: Mandolin.
That day Aang was babysitting his little army of nephews, there were in Republic City, the whole Avatar team was visiting, and they were all meeting again after so long; however, that day Aang was left taking care of his small army of nephews, from Bumi who had just turned fifteen years old to little Suyin who was barely a year old baby girl.
Aang looked how Kya played with his twins, while Bumi was lost somewhere in the house and he was trying to get Suyin to take a nap, he knew that this was some kind of revenge from his two friends and maybe his husband, because it seemed that the kids preferred to spend more time with him than with any other adult although he preferred not to think about it too much and enjoy all his kids, he continued to lullaby the baby while humming his special lullaby, one that he had known from a very young age and still sang to his children. When he finally managed to get Suyin to nap, he set out to see her twins and Kya to make sure they were not doing anything dangerous; however, Bumi finally appeared carrying a mandolin in his hands and seemed somewhat nervous so Aang approached to him.
"Uncle, I want to show you something" the teenager said when he was in front of his uncle, Aang just nodded making the young boy smile.
Bumi took Aang as far as he could from his younger sister and sat by a tree to prepare himself before playing, he took a deep breath and began to play the instrument, although Aang did not fully recognize the song at first, after a few seconds, the melody started to sound a bit different. He knew it was the lullaby he sang to everyone when they were toddlers, it was the song he sang to himself when he was scared and when he began his adventure to end the Hundred Years War. That song was played for him by monk Gyatso when he could not sleep at night, when he was still very young and Bumi's way of playing was the same as his old tutor did.
Aang began to hum the song that his nephew was playing, and he saw that Bumi had relaxed a little, until almost the end of the song came, the twins came up followed by Kya; when the teenager finished, he looked a little embarrassed to see his cousins and sister there.
"You play well" Izumi said once she arrived and sat next to Bumi "that's how my mom song sounds, it's cute"
"Could you play it again?" Tenzin asked this time sitting next to his sister, while Aang made Kya sit next to him and prevented him from making any sarcastic comment.
Bumi nodded and started again but this time the twins sang the lyrics that Aang had made for them and even Kya was humming the song, because Aang had made that song for his family and those children, even if they were not his own, he almost felt them like that.