Maybe in the fanfiction, Voldemort's return in a different way causes the wizarding government to spend all its funds on defense. They might have to buy expensive magical weapons, hire an army of aurors, and build fortified defenses. This over - expenditure could lead to bankruptcy not just of the wizarding part of Britain but also have a domino effect on the non - magical part as the two economies are likely connected in some ways. Also, the fear of Voldemort could stop all trade and tourism, further worsening the situation.
A scenario could be that a new, extremely powerful magical being emerges in the fanfiction who demands huge tributes from Britain. To meet these demands, the government has to sell off all its assets, nationalize private companies, and raise taxes to astronomical levels. This could lead to businesses shutting down, people losing their jobs, and eventually the bankruptcy of Britain. Another aspect could be that the magic used to keep the wizarding world hidden fails, and the sudden revelation causes mass panic and economic turmoil.
Well, perhaps in this fanfiction, Harry Potter accidentally casts a spell that has a domino effect on the magical economy of Britain. For example, he might mess up a powerful charm related to the wizarding currency, leading to a rapid devaluation. Or maybe he uncovers some dark secret about the Ministry of Magic's financial management that causes a mass panic and economic collapse.
It could be a corrupt wizard within the Ministry of Magic's financial department. He might be embezzling funds and using dark magic to cover his tracks. When Harry starts to dig into the strange financial situations in Magical Britain, this wizard tries to stop him by all means, which eventually leads to the bankruptcy as the situation gets out of control.
One common scenario is that Harry leaves Britain and goes to live in another wizarding community. He might feel that he can start anew away from the pain and memories of Britain.
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.
Well, perhaps Harry discovers some major financial mismanagement in Gringotts. Maybe he finds out that the goblins have been embezzling funds or making really bad investments. This could lead to a situation where the bank is on the verge of collapse and Harry, being the hero he is, decides to expose the truth and in the process, bankrupts the bank as all the secrets come to light.
Perhaps in the fanfiction, Harry Potter runs away from Britain because he is disillusioned with the way the Ministry of Magic operates. There could have been a major event where the Ministry fails him or betrays his trust. Also, he may be seeking a place where he can explore his magic without the constraints and expectations that come with being 'The Boy Who Lived' in Britain.
One possible plot could be that Harry discovers a dark secret about the Ministry of Magic, like they are using dark magic themselves to control the wizarding population. So he decides to expose them and in the process, the chaos that ensues leads to the destruction of magical Britain as it was known.
One scenario could be that Harry uses his invisibility cloak to help the Cullens spy on their enemies without being detected. Another is that he could use the Time - Turner (if it exists in the fanfiction world) to go back in time and prevent something bad from happening to the Cullens.
Well, one possibility could be Itachi as a wizard in the Harry Potter world. Maybe he'd be in Slytherin due to his cunning nature. He could use his Sharingan abilities in a magical way, like seeing through invisibility cloaks or detecting hidden spells.
One possible scenario is that Harry Potter helps the giants when they are being hunted by dark wizards. He uses his invisibility cloak and other magical items to hide the giants and lead them to safety. Another scenario could be that he helps the giants who are sick. He might search for rare magical herbs to cure their diseases.