It could be that he is magically whisked away by a mysterious force. Perhaps there is a long - lost family member or an ancient magic that has been awakened on his 16th birthday and it takes him to a far - off place. There he has to learn new magic and skills to be able to return and face the challenges in his world.
Maybe he discovers a dark secret about his past and decides to run away to protect his friends and Hogwarts. He could have found out that Voldemort has some new evil plan related to him specifically on his 16th birthday.
He might disappear because of a powerful spell gone wrong. On his 16th birthday, he could be trying out a new spell that he thought was harmless but instead, it teleported him to an unknown location. This could lead to a series of adventures as he tries to find his way back, all while his friends are desperately searching for him.
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.
He might end up in a place where he can't see clearly without his glasses and struggles to find his way. For example, he could be in a dark forest where he stumbles around, trying to figure out where he is and how to get back. Without his glasses, his magical abilities might also be affected in some way, like his spell - casting accuracy.
One possibility is that Naruto is sent on a top - secret mission by ANBU. Maybe he has to infiltrate a powerful enemy organization. His disappearance could be part of a long - term undercover operation where he has to cut off all ties with his friends and the village for a while to avoid suspicion.
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.
Well, it could start with Ginny waiting all day for Harry to remember. When he doesn't, she gets angry and ignores him. Harry, being a bit clueless, doesn't understand why she's so mad until Hermione smacks him on the head (figuratively) and tells him. Then he might plan a surprise party in the Room of Requirement with all of Ginny's favorite things, like butterbeer and Weasley sweaters in different colors.
In fanfiction, if Harry Potter disappears, it could lead to chaos at Hogwarts. His friends like Hermione and Ron would likely be on a desperate search for him. They might suspect the Dark forces at work again, and start investigating all the possible places where Harry could have been taken or hidden.
It could be Death Eaters. They might have finally found a way to capture Harry without alerting Dumbledore. His 16th birthday could be the perfect opportunity as everyone might be a bit more relaxed thinking he's not quite of age yet for the full - on final battle.
One possibility could be that Naruto is kidnapped by a hidden enemy right at birth. Maybe a rogue ninja group has a vendetta against the Fourth Hokage and they steal Naruto away. This would lead to a completely different story arc in the Naruto universe, with Konoha perhaps not knowing Naruto's true identity and not treating him as the Jinchuriki. It could also mean that Naruto grows up in a different environment, away from the discrimination he faced in the original story, but also without the friends and support system he eventually had in Konoha.
Voldemort could be the antagonist. Even if he was defeated in the original story, there could be a scenario where some remnant of his dark magic causes Harry and Hermione to disappear. Maybe he had a secret plan that was set to activate after his death and it involved taking them out of the picture.
It could be a new dark wizard who has been secretly plotting in the shadows. This wizard might be jealous of Harry's fame and power, and decides to make him disappear so that he can rise to power unopposed. He could have a group of followers who are experts in dark magic and are able to carry out his plan to make Harry vanish.