One possible plot could be that Thor ends up in the wizarding world of Harry Potter. Maybe the Bifrost malfunctions and he lands near Hogwarts. He could be mistaken for a very powerful wizard at first. Then he teams up with Harry Potter and his friends to fight against a new dark threat that is using both magic and some form of Asgardian - like power. Thor could teach the wizards about Asgardian combat and they could teach him more about the nuances of magic.
The plot might involve a magical object in the Potterverse that has a connection to Asgard. Thor is sent to retrieve it. He meets Harry and they initially clash because of their different ways of doing things. But then they realize they need each other. Thor's strength and lightning powers combined with Harry's magic and knowledge of the wizarding world could be used to defeat a villain who wants to use the object to gain ultimate power over both worlds.
It could be Loki. Since he has a knack for causing trouble, he might find a way to cross over to the wizarding world. He could manipulate the dark wizards and use their magic for his own devious plans. Maybe he wants to steal the Philosopher's Stone to gain immortality and use it to take over Asgard as well.
One possible plot could be that Thor somehow gets transported to the wizarding world. He might initially be confused by the magic there as it's different from his Asgardian powers. Harry Potter and his friends could be intrigued by Thor and his hammer. They might team up to fight a common enemy, perhaps a dark wizard who has somehow got hold of some Asgardian artifact and is misusing its power.
One common theme is the clash of cultures. Harry comes from a world of wizards with their own set of rules and traditions, and Thor is from Asgard with its warrior culture. Another theme could be the exploration of power. Harry has his magic, and Thor has his godly strength. Writers often like to see how these two forms of power interact.
A powerful dementor - like being that has been corrupted by Asgardian dark magic. This entity could be draining the life force of both wizards and Asgardians. It would be up to Thor and Harry Potter to find a way to stop it, as it poses a threat to both of their worlds.
Well, it could be that Harry Potter uses his magic skills like the Patronus charm against Thor's lightning. Maybe Thor mistakes Harry's magic for some new Asgardian sorcery at first. And then they start to fight over some misunderstanding, like a magical artifact that Thor thinks is from Asgard but Harry knows is important for the wizarding world.
Yes, there are quite a few out there. You can start by looking on fanfiction websites like Archive of Our Own. Just search for 'Harry Potter x Thor Odinson' and you'll get a list of stories. Some might be crossovers where Harry and Thor meet in a magical world, while others could have more creative plotlines like Thor visiting Hogwarts or Harry traveling to Asgard.
One possible plot could be that the Halliwell sisters from 'Charmed' somehow get transported to the wizarding world of 'Harry Potter'. They could use their powers to help Harry and his friends in a battle against Voldemort. For example, Piper could freeze Death Eaters with her time - stopping power, while Phoebe could use her premonition power to foresee Voldemort's next moves.
Maybe there's a prophecy in both worlds that foretells of a combined force from the wizarding world and the ninja world being the only way to defeat an ultimate evil. So, Naruto and Harry are brought together. They face challenges in understanding each other's cultures and powers at first. But as they train together, they find ways to merge their skills. For instance, Naruto's rasengan could be enhanced with a magical spell to create a more powerful attack.
A possible plot is that a powerful magical artifact in the wizarding world is actually a key to an ancient Stargate on Earth that has been long forgotten. Harry and his friends, along with the Stargate team, embark on a quest to find this Stargate. Along the way, they face challenges from dark wizards who also want the artifact for their own evil purposes and the Goa'uld who sense the power of the artifact. They must work together, using both magic and technology, to reach the Stargate and unlock its secrets.
One possible plot could be that Gordon Ramsay visits the Hogwarts kitchens. The house - elves are used to their traditional ways of cooking and are resistant to change. Ramsay tries to introduce modern muggle cooking techniques to them. For example, he might show them how to make a proper steak instead of just serving up mystery meat stew all the time. There could be chaos as the house - elves initially reject his ideas, but eventually come around when they see the students enjoying the new food.