It's clear that Harry and Daphne are the main characters. Harry brings his courage, skills, and a network of friends (even if they're leaving England). Daphne may bring a different perspective, perhaps she has a more academic or cultural knowledge that will help them in the new places they'll visit. Their relationship and how they interact during this departure from England will likely be a major focus of the fanfiction.
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.
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 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.
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.
Perhaps they would head to America. In the wizarding world of America, there are different magical institutions and a large expanse of land where they could start anew. They might be drawn to the Ilvermorny School of Witchcraft and Wizardry area, to interact with new wizards and witches and learn from a different magical education system. Also, the American magical community might not be as involved in the affairs of the English wizarding world, providing them with some anonymity.
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.
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 challenge could be transportation. If they are trying to leave secretly, they can't just use the regular wizarding transportation like the Hogwarts Express or Portkeys that are easily monitored. They might have to find some unregulated or hidden means of travel, like an old, enchanted flying vehicle that is difficult to control but less likely to be traced.
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.