In this fanfiction, it could be that Harry Potter has a vision of a future where Voldemort has completely taken over the wizarding world and killed all his loved ones. This drives him into a state of madness and he goes on a rampage. He travels back in time using some ancient magic he discovers, and starts to take out all the Death Eaters one by one, not caring about the consequences, as he believes it's the only way to change the future.
It could be a new dark wizard who has some personal vendetta against Harry. Maybe this wizard has a grudge because Harry defeated Voldemort and he wants to prove he's more powerful, which enrages Harry and causes him to go on a rampage.
In the 'naruto goes on a rampage fanfiction', it could be that Naruto is under some sort of dark magic or mind - control. This makes him lose his usual self - control and go on a rampage. He could start attacking both enemies and allies without realizing it until someone finds a way to break the spell on him.
Well, perhaps Harry Potter goes to Dalton and discovers a whole new world of magic there. Maybe he meets new friends and has to face different challenges compared to Hogwarts.
One possibility could be that after facing too many dark magic encounters, Harry's mind starts to break. Maybe the constant threat from Voldemort and the losses he endures, like the death of his friends, push him over the edge. He could start to have hallucinations and lose his ability to distinguish between reality and dark magic-induced illusions.
In a 'Harry Potter goes to jail' fanfiction, perhaps Harry breaks a new law that the Ministry of Magic has created to maintain control. For example, if he tries to teach magic to non - magic people in an attempt to prepare them for a potential dark wizard threat. The Ministry, fearing chaos, throws him in jail. Inside, he meets other wrongly - imprisoned wizards and they plan an escape together while also fighting against the unjust system.
Maybe it's about Harry Potter being framed for using dark magic in a way that violates the wizarding laws. As a result, he has to go to this 'juvie' place. In there, he discovers that there are other kids who have also been wrongly imprisoned. He then uses his skills and the loyalty of his friends (who might try to break him out or help him legally) to fight against the unjust system.
In this fanfiction, perhaps Harry goes to Azkaban to rescue someone. It could be a fellow wizard who has been unjustly imprisoned. He might have to use his skills and some new-found magic to navigate the dangerous and dreary corridors of Azkaban, all while avoiding the Dementors.
One possible plot could be Harry struggling to fit in at Harvard. He might be the only wizard there, so he has to hide his magical abilities while also trying to keep up with the intense academic curriculum. For example, he could use his quick thinking skills from his Hogwarts days to solve complex problems in his Harvard classes.
Well, perhaps Harry could be on a quest to find some ancient magic artifacts in Egypt that are related to Hogwarts' history. Maybe he discovers a hidden connection between Egyptian magic and the magic he's learned at school.
Maybe Harry Potter goes to Africa to study some unique magic that only exists there. He could encounter new magical creatures and make friends with African wizards.