One possible plotline could be Harry leaving Britain to explore the magical communities in other countries like France or America. He might discover new spells or magical creatures there.
In some fanfictions, Harry leaves Britain because he feels betrayed by the Ministry. He could then start a new life in a hidden magical enclave, away from the politics and drama of his homeland. There, he might focus on honing his own unique magical abilities that he hadn't had time for before. For example, he could study ancient forms of magic that are long forgotten in Britain. He may also make new friends who are not influenced by the prejudices and power struggles of the British wizarding world.
One popular 'Harry Leaves Britain' fanfic could be about him traveling to a far - off magical land to study new kinds of magic. For example, he might go to a hidden school in the Amazon rainforest where magic is intertwined with nature in ways he's never seen before.
One possible plotline could be that Harry discovers a hidden magical portal in Hogwarts that leads outside of Britain. Another could be that he fakes his own death and secretly boards a ship bound for a far - off magical land to escape the danger in Britain. Maybe he teams up with some international wizards who help him smuggle out of the country under the guise of a diplomatic mission.
One interesting scenario could be Harry Potter starting a new life in the Muggle world and using his magical skills in secret to help people. For example, he might use magic to heal the sick in a Muggle hospital without them knowing. Another could be him exploring different magical cultures outside of Britain, like those in Asia or Africa that are not much explored in the original books. He could learn new spells and magical techniques and bring them back to the wizarding world in the future.
There's a fanfic called 'Farewell, Britain'. Here, fem Harry leaves Britain because she feels that she can no longer tolerate the false accusations and the dangerous situations she's constantly in. She heads to America where she discovers a whole new world of magic. It's full of adventure as she meets new friends and foes alike. It also explores her growth as a person outside the familiar British magical world.
Well, in some 'harry potter king of magical britain fanfiction', there's a plotline where Harry has to deal with dark wizards who oppose his rule. He uses his unique skills and the loyalty of his friends to fight them off. Also, there could be a story where he uses his position to improve magical education, perhaps by bringing in new subjects that were previously ignored, like advanced defensive magic for all years at Hogwarts.
One possible plotline could be Harry using magic in creative ways to evade the authorities as he escapes. For example, he might use a complex series of apparition spells combined with disillusionment charms.
One common theme is self - discovery. When fem Harry leaves Britain, she often has to figure out who she is outside of the expectations and restrictions of the British wizarding world. For example, in many stories, she realizes she has different magical abilities or interests that were suppressed in Britain.
Voldemort or his followers could also be relevant in some stories. Maybe Harry is leaving because he fears a resurgence of Voldemort's forces and wants to find a way to defeat them from afar. Or perhaps some of Voldemort's old followers are still after him, and his leaving is a way to draw them away from the innocent people in Britain.
There's 'The Journey of Fem Harry Away from Britain'. In it, Fem Harry decides to leave Britain as she believes she can't grow and fight Voldemort effectively in the environment she's in. She may encounter new allies and enemies on her journey away from Britain. Maybe she meets some foreign wizards who have different ways of magic that could help her in her fight against Voldemort. And then there's 'Fem Harry's Departure'. This fanfic could explore how her leaving Britain is a strategic move to draw Voldemort out of his comfort zone and into a new battlefield.
They might write such fanfics to give Harry a new adventure. After all the events in the original books, his story doesn't have to end in Britain. It's a way to expand on his character and see how he would react in different situations outside his familiar homeland.