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.
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.
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.
Well, one interesting plot point could be how Harry's personality changes under the influence of the founders. Maybe he becomes more in tune with their values and ways of magic. Another could be the conflicts that arise within him as the different founders' spirits or essences vie for control.
Well, 'Shadows in Harry's Soul' is a fanfic that's quite good in this regard. It starts with a slow build - up of Voldemort's influence over Harry. As the story progresses, we see Harry's attempts to break free from the possession. The fic also has some interesting sub - plots involving the other characters trying to figure out what's going on with Harry and how to save him.
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.
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.