Perhaps Harry Potter is summoned to Queen Elizabeth's court by a dark wizard who wants to use Harry's fame to gain power. Harry has to find a way to escape this situation and also protect the queen from the dark wizard's plans. He might use his knowledge of magic and his Gryffindor courage to outwit the dark wizard and return to his own time, leaving behind a legend in Queen Elizabeth's court.
One possible plot could be that dark wizards are planning to use Queen Elizabeth as a symbol to gain power in the Muggle world. Harry Potter, being the hero, gets wind of this plot and uses his magic to secretly protect her. He could use spells like the Fidelius Charm to keep her location hidden from the dark wizards.
Harry would likely be very curious about the court and its people. He might use his magic in small, discreet ways to amuse the courtiers, like making a flower bloom in his hand. He would probably be polite and respectful, but also a bit of an outsider. His different style of dress and his magic would make him stand out, and he'd have to explain himself carefully to avoid being seen as a threat.
It could be a very interesting fanfiction. Maybe Queen Elizabeth could be a secret witch in the Harry Potter universe. She could use her magic to rule Britain in a different way, perhaps with the help of Hogwarts' magic. For example, she could use spells to improve the economy or protect the country from dark wizards.
It could be about Queen Elizabeth somehow being a part of the wizarding world in the 'Harry Potter' series. Maybe she's a secret witch or has some connection to Hogwarts.
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.
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.
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.