It could be that Aang disappears because of a prophecy. He might believe that by disappearing, he can prevent a catastrophic event from happening. His friends, not knowing this, embark on a journey to find him, facing many challenges along the way. They might discover secrets about Aang's past that they never knew before, and these secrets could be the key to bringing him back.
A secret society that has been observing the Avatar from the shadows could be the main villain. They believe that Aang's power is too great and unpredictable, so they use some ancient and powerful artifacts to make him disappear. The heroes would then have to figure out how this society operates and find a way to reverse the spell or device that made Aang vanish.
The setting in which Harry disappears and returns also matters. If it's in the middle of Hogwarts, the school would be in chaos. Teachers would be trying to keep the students calm while secretly searching for Harry. And when he returns, the reactions of different houses - Gryffindor being relieved and proud, Slytherin perhaps being suspicious - could add more layers to the story.
I'm not entirely sure specifically what 'ash disappears fanfiction' is about without more context. It could potentially be a fan - made story in any number of fandoms where a character named Ash goes missing. It could explore themes of loss, mystery, or the reactions of other characters to this disappearance.
Well, 'sakura disappears fanfiction' could be about a story where sakura, which might refer to cherry blossoms in a Japanese - themed context, disappears. It could be a fictional account of a world where these beautiful flowers vanish for some mysterious reason, like a magical curse or an environmental disaster in the fictional setting.
He could meet his future self. It would be an interesting encounter as he could learn from his future experiences and wisdom. Or he might meet people who have studied his past adventures and see himself through their eyes.
Perhaps Harry is kidnapped by a mysterious and powerful magical being. This being has the ability to make Harry disappear without a trace. His friends then embark on a dangerous quest to find him, facing many magical challenges along the way. They might have to search in hidden and dangerous parts of the wizarding world that they have never been to before.
Possibly they are random villagers who were caught in the crossfire of a conflict between different bending factions. Aang, being the Avatar and a symbol of peace, feels responsible for burying them and ensuring their spirits are at peace.
He might start by observing. Aang is a curious person, so he'd likely watch how people live, how they interact with technology. He may also try to find other benders to learn about any new developments in bending. And he'd probably use his kind heart to make friends easily in this new future environment.