It could be really interesting. Maybe it would involve a world where magic from both universes exists. They could use their combined powers to solve mysteries or fight against common enemies.
There could be. Fanfiction writers come up with all sorts of creative ideas. However, it might not be a very common theme compared to more mainstream fanfiction ideas in those particular fandoms. You might have to dig a bit deeper into the archives of the fanfiction communities to find one. You could also join relevant fan groups on social media platforms and ask if anyone knows of such a fanfiction.
A 'harry potter is ciel phantomhive fanfiction' could involve a lot of world - building combinations. Maybe the Death Eaters in Harry Potter could be related to the demons in Black Butler. Hermione could use her knowledge to help Ciel solve some of his business problems in the underworld. The story could also explore how the magic in Harry Potter interacts with the supernatural elements in Ciel's world, like the contracts Ciel has with demons. It would be a complex and creative mix of two very different fictional elements.
It would be a really interesting mix. Harry comes from a world of magic with wizards and witches, while Yugi is in a world of card games with ancient Egyptian magic. If they were brothers, maybe they would combine their powers. Harry could use his magic to enhance Yugi's card battles, or Yugi could use his knowledge of the Millennium Items to help Harry in his fight against Voldemort.
It could be very interesting. There might be a lot of emotional turmoil as they discover their relationship. Harry would have to reevaluate his feelings towards Snape, and Snape might have to come to terms with having a family connection to Harry.
It would be very strange and interesting. It might involve a complete rewrite of the backstories. Maybe there would be a hidden family connection that was somehow overlooked in the original story. Harry's character might have some of Bellatrix's traits, like her fierceness or her loyalty (albeit to different sides in the original story).
It could be a very strange and interesting story. Harry might have some sort of divine powers in addition to his wizarding abilities. Maybe he could perform miracles or have a special connection to a higher power that helps him in his battles against Voldemort.
In such a fanfiction, there would be a lot of cultural clashes and interesting interactions. Harry, being raised in the wizarding world, would have to adapt to Asgardian customs and vice versa for Thor. The story could explore how their different upbringings shape their personalities. Also, their combined powers could be a great asset in battles. For example, Harry could use magic to enhance Thor's lightning attacks or Thor could use his strength to protect Harry during a wizard duel. It would be a story full of action, magic, and brotherly bonding.
It could be really exciting. Maybe Harry would use his magic in Gotham City to fight crime alongside Batman. Or perhaps they'd discover their relation through some mysterious magical - technological means. Their different skills and abilities would make for an interesting combination in the story.
Well, in such a fanfiction, it could start with Harry discovering that he has a brother in Sherlock. The plot could involve them coming together and realizing how their different worlds - the magical world of Harry Potter and the detective - filled world of Sherlock - can complement each other. For instance, Harry could use his invisibility cloak to help Sherlock in stakeouts. And Sherlock could use his powers of observation to help Harry in the wizarding world, like noticing if someone is under a charm or a spell. It would be a unique blend of two very different universes.
In such a fanfiction, there could be a whole new set of adventures. Since Harry is from a world of magic and Naruto from a world of chakra, they could find a way to combine their powers. The story might start with the discovery of their relation. Then they could go on a journey to find out more about their family's past and why they were separated. Along the way, they would face many challenges, like enemies from both worlds who don't want to see them united.