I came up with a fic to go with that line after all! I’m going to put this up on ao3, but because it’s being a butt at the moment I’ve put the whole thing here too. Warning: this will give you a serious, serious case of the sads. It bummed be out incredibly while I was writing it.
Sometimes Tenzin feels as if he is simply an empty vessel in the shape of his father, waiting to be filled. He stands at the altar, silent, his brother and sister beside him. Katara hides her face in her hand and sobs.
Aang teaches him the rituals before he dies. There is no-one else.