One idea could be that Harry would use his magic skills to help the Fellowship in different ways. For example, he could use his Patronus to ward off the Nazgûl, as his Patronus is a powerful form of positive magic. Another possibility is that he would have an interesting interaction with Gandalf, two powerful wizards from different magical worlds coming together.
Well, he could use his magic to help the Avengers in battles. Maybe he could use a spell to disarm Thanos of the Infinity Stones. His broomstick flying skills would also be useful for quick getaways or aerial attacks.
One idea could be a crossover where the characters from Harry Potter are transported to Middle - earth. For example, Harry could meet Aragorn and they could exchange knowledge about magic and leadership. Harry's magic could be a new and strange power in Middle - earth, and Aragorn's warrior skills and wisdom could inspire Harry.
In a fanfiction scenario, Harry Potter in the Game of Thrones world might be seen as a threat by the maesters. They might try to either study him or get rid of him. Harry, on the other hand, could form alliances with some of the outcasts like the Wildlings. He could teach them some basic magic for self - defense, which would change the dynamic of their relationship with the rest of Westeros.
One idea could be a crossover where the characters from 'Harry Potter' end up in Middle - earth from 'Lord of the Rings'. For example, Harry Potter could meet Frodo and they could exchange their experiences of facing great evil. Another idea might be a fanfiction where the magic systems of both worlds are combined, like using a Hogwarts spell to defeat a Mordor beast. Or perhaps a story where Aragorn visits Hogwarts and learns about the different forms of magic there.
In a Harry Potter fanfic with the Infinity Stones, the Stones could interact with the existing magical artifacts in interesting ways. The Elder Wand, for example, might have some sort of connection or reaction with the Infinity Stones. Wizards and witches who were already powerful, like Dumbledore, would have to figure out how to deal with these new and extremely powerful elements in their world. It could also lead to new forms of magic being discovered as the Stones' powers merge with the magic already present in the Harry Potter universe.
His presence could speed up the journey of the Fellowship. Harry's magic could be used to quickly cross difficult terrains or overcome obstacles that the original Fellowship had to struggle with. For example, he could use a spell to create a bridge over a chasm instead of having to find a way around it.
He'd probably have a more comical approach to magic. Mystery Inc. has its humorous moments, and that could influence how Harry uses magic. Instead of just using spells for serious things, he might use them in a more fun and creative way, like creating magic tricks to amuse his friends or to outwit his enemies.
Well, the wizards would surely have some unique ways to deal with zombies. For example, they could use spells like 'Incendio' to set the zombies on fire. And the protective charms around Hogwarts might keep the zombies out at first. The students would probably be trained to fight them in Defense Against the Dark Arts classes.
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.
In fanfiction, if Harry Potter becomes Lord, he might gain a great deal of power and influence. He could use this new status to reform the wizarding world. For example, he could change the way Hogwarts is run, introduce new magical studies, or even restructure the Ministry of Magic to be more just and efficient.