In a fanfic where Harry Potter is the King of Britain, he could revolutionize the legal system. Since he has seen the flaws in the wizarding world's laws, he would create new laws that are more just. He might also use his influence to promote the study of ancient magic. Harry would use his position to bring together different magical communities within Britain, such as the centaurs, the goblins, and the house - elves, and give them more rights and representation in the kingdom.
If Harry Potter were emperor in a Mass Effect fanfic, it would be a wild ride. He would have to adapt to the technology of Mass Effect while also using his magic. His knowledge of defensive spells could be used to fortify planets or spaceships. Also, his ability to see thestrals, which is related to death, might give him an edge in understanding the more dangerous aspects of the Mass Effect universe, like the Reapers' true nature.
He might be quite amused. Fanfics often contain different interpretations of the characters and the wizarding world. He could find some creative ideas in them that he never thought of before, like unique magical spells or new backstories for his friends.
He might use his ninja skills to bring a new dynamic to the DC universe. For example, his shadow clone jutsu could be used in battles or for strategic purposes, like gathering information simultaneously in different locations. His rasengan could be a powerful attack against DC villains.
In a 'Harry Potter marries a king' fanfiction, it would likely be a story full of adventure and discovery. Maybe the king discovers the wizarding world through Harry. They could go on quests together to protect both the Muggle and wizarding realms. Harry could introduce the king to magical creatures and spells. Their marriage would be a symbol of unity between two different worlds. But they would also face opposition. Some in the wizarding world might think it's wrong for Harry to marry a Muggle - like figure, and some in the king's court might see Harry as a threat to the traditional order.
Well, he could bring his magic skills to the team. Maybe he could use spells to enhance the Avengers' weapons or defenses. For example, he could put a protection charm on Iron Man's suit to make it even more resistant to attacks.
In such a fanfic, Harry might experience extreme pain and weakness. His friends would likely be in a panic. Hermione would probably immediately start looking for antidotes in the library or with her vast knowledge of potions. Ron would be by Harry's side, trying to keep him conscious and reassuring him.
In such a fanfiction, Harry might grow up with a very different set of values. King Artoria is known for her chivalry and strong sense of justice. Harry could be trained in combat and knightly virtues from a young age. He might also have a different view on magic, perhaps more in line with the noble and traditional ways that Artoria represents.
In such a crossover, perhaps Harry Potter could use his magic in Camelot. He might help King Arthur fight against dark magic threats that Camelot faces, like Morgana's sorcery. His spells could be a new weapon in Arthur's arsenal.
In a fanfiction scenario where Harry Potter fights the Night King, it could be a very interesting battle. Harry has his magic skills like the Patronus charm which could potentially be used against the Night King's army of the dead. However, the Night King is extremely powerful and has the ability to raise the dead. Harry would need to find a way to counter this, perhaps by using powerful offensive spells like the Killing Curse. But he would also have to be careful not to get overwhelmed by the sheer number of the Night King's forces.
Well, if Harry Potter became king in fanfiction, Hogwarts might become a central part of the kingdom. Magic could be used to improve the lives of ordinary people. He would probably use his Gryffindor courage to make fair and just laws. And his friends, like Hermione and Ron, would likely be his advisors, bringing their own skills and knowledge to help him rule.