One interesting 'Harry Potter Scared Straight' fanfiction could be one where Harry is sent to a wizarding reform school after getting into trouble at Hogwarts. In this fanfic, he meets characters who have really gone off the rails and they tell him their horror stories to scare him into being a better wizard. It might show how he has to face his own dark tendencies and overcome them.
A possible 'Harry Potter Scared Straight' fanfiction might involve the ghosts of Hogwarts. The Bloody Baron or other spooky ghosts could be given the task of scaring the students into following the rules. Harry, being the curious one, gets involved in a mystery related to this. He discovers that the ghosts have a deeper purpose than just scaring them, and it could be a story about redemption and learning from the past mistakes of those the ghosts represent.
There could be a 'Harry Potter Scared Straight' fanfiction where the Dementors are used in a different way. Instead of just guarding Azkaban, they are brought to Hogwarts to scare the students straight. Harry has to deal with their presence and the fear they bring, and this experience makes him more determined to fight against dark magic. It could also explore how the other students react to this extreme measure.