One possible plotline could be Harry struggling to regain control of his mind while Voldemort tries to use his body to carry out evil deeds. For example, Harry might have to fight against Voldemort's commands internally. Another could be how his friends notice the strange behavior and try to figure out what's wrong and save him.
In some fanfictions, they might rewrite the possession scene completely. Instead of the typical way it was shown in the books, they could add new magical elements. Maybe Quirrell has some hidden magic that reacts to the possession, creating a more complex and engaging story.
One common theme is the struggle for control. Harry, while possessed, battles with the entity inside him for control over his body and actions. Another theme is the isolation it causes. As he is different and acting strangely, he often gets isolated from his friends, like in many fanfics where Hermione and Ron are initially distrustful of the possessed Harry.
There are fanfictions where Voldemort tries to possess Harry in a different way than in the original books. For example, instead of through a simple curse, he uses a complex magical artifact. This possession changes Harry's personality gradually, and his friends have to find a way to save him. In these fanfictions, the author often explores the psychological effects of possession on Harry, which is quite fascinating.
The other Hogwarts students would be a mix of scared and curious. Some might avoid Harry altogether, fearing that they could be in danger. Others, who are more brave or naive, might try to approach him to see if they can help. The teachers would be in a difficult position. They would need to protect the other students but also try to find a solution for Harry. For instance, Professor McGonagall might be torn between her loyalty to Harry and her duty to keep the school safe.
Another one is 'Possessed Potter's New Path'. This fanfic imagines Harry being possessed by a mysterious entity that gives him a whole new perspective on the wizarding world. It has some unique magical concepts and shows a different side of Hogwarts as Harry, under possession, interacts with the school and its inhabitants in unexpected ways.
In addition to the trio, there could be characters like Professor McGonagall. She would surely notice the changes in Harry and might try to guide him. Also, the ghosts of the founders themselves could be considered main characters in a sense. Since Harry is possessed by them, they would have a significant influence on the story, perhaps communicating with Harry or causing events to unfold around him.
Check out blogs or websites that are dedicated to reviewing 'Harry Potter' fanfictions. These often categorize the fanfictions based on different themes, and you can easily find the ones related to possession. Some of these blogs might also provide in - depth analysis of the fanfictions, which can help you decide whether a particular one is worth reading.
There are various fanfics with this theme. 'A Girl Possessed by Magic in Harry Potter's World' is another one. In this fanfic, the girl, who is possessed, discovers unique magical abilities that are not part of the normal Hogwarts curriculum. It shows how she uses these newfound powers to interact with the characters we know from the original series, like Hermione and Ron, and how they react to her strange situation.
Another common plot element is the struggle. The girl has to deal with being possessed. This could involve trying to control the foreign entity within her or trying to figure out why it happened. In some fanfics, she might be in conflict with dark forces that are using the possession to their advantage, while in others, she's just trying to get her normal life back while also dealing with this new magical situation.