It could be that Harry and Susan are escaping some sort of danger in England. They leave secretly and head to France. There, they try to blend in with the local community. Harry might find work as a painter and Susan could start teaching English. Their journey would be about not only starting anew but also constantly looking over their shoulders in case the danger from England follows them.
They could be two ordinary friends who have just decided that they need a change in their lives. Maybe they've grown up together in a small English town and now want to see what the world outside England has to offer.
One possibility is that they leave England in search of a magical place that can offer them a peaceful life. They could be tired of the constant battles and power struggles in England. They might go to a far - off land where magic is used in a different, more harmonious way. For example, they could head to some hidden island where ancient magic thrives and they can study and grow without the interference of the Ministry or dark wizards.
One possible reason could be to protect Teddy. Maybe there are some threats in England that Harry wants to keep Teddy away from. For example, there could be some dark wizards or dangerous magical creatures that target Teddy for some reason.
One possible reason could be to escape the danger and chaos that might be brewing in England within the wizarding world. Maybe there are dark forces targeting them specifically, and leaving is their only option for safety.
One reason could be to find peace. After all the chaos and war in England, they might just want to get away from it all and start fresh somewhere else.
Maybe Harry leaves England to explore the wizarding world in other countries. He could go to France to study different magic spells at Beauxbatons Academy. Or perhaps he travels to America to help the MACUSA (Magical Congress of the United States of America) deal with some dark magic threats.
One possible plot could be that Harry discovers he has some long - lost royal blood. He then has to navigate both the wizarding world and the political intrigues of the English court as he tries to balance his newfound status as a potential king. Magic could be used to solve problems within the monarchy, like protecting the king from dark magic threats.
One possible plot could be that Harry Potter discovers his secret heritage as a Prince of England. This might lead to a whole new set of magical and political adventures. He could use his newfound status to reform the wizarding world's relationship with the Muggle government. Maybe he'd introduce magical solutions to some of the Muggle world's problems, all while still dealing with the usual threats in the wizarding world like Voldemort's followers.
Well, it could be about the Queen of England being a secret witch and having a connection to the wizarding world of Harry Potter. Maybe she helps Hogwarts in some way during a crisis, like providing a safe haven on the castle grounds that are protected by magic and Muggle secrecy charms at the same time.
In the fanfic, Sirius and Harry leaving England could lead them to a hidden magical island. There, Harry could learn different kinds of magic that are not taught at Hogwarts. Sirius would be able to protect Harry better as they are far away from the threats in England. They might also start a new life where Harry can grow up without the constant fear of Voldemort's followers finding him.