The plot could involve Harry Potter being reborn in the Vampire Diaries universe with his magical abilities intact. He might face new challenges like dealing with vampires and witches in this different magical setting. Maybe he'd use his spells to protect himself from vampire attacks or form alliances with some of the characters. For example, he could team up with Bonnie Bennett, sharing magical knowledge. And he'd surely have to adapt to the different social structures and power dynamics in Mystic Falls.
Well, it could be that Harry's reborn presence disrupts the balance in the Vampire Diaries world. The vampires there might see him as a threat or an opportunity. He could introduce new forms of magic that the witches in the Vampire Diaries haven't seen before. And his experiences with fighting dark wizards in his original world could give him an edge in dealing with the various villains in the new setting. Also, his loyalty and sense of justice would likely lead him to get involved in the local conflicts.
Harry's magical abilities would be adapted to the new environment. His knowledge of defensive magic would be crucial. He could use the Shield Charm in new ways, like protecting against vampire compulsion. His flying skills on a broomstick might not be as useful as in Hogwarts, but he could learn to use magic to levitate himself quickly to avoid vampire attacks. Also, his ability to sense danger, which he developed from his experiences in Hogwarts, would be heightened in this more dangerous Vampire Diaries world.
Well, it could be that Sakura, with her knowledge from her previous life, might bring some unique magic from her own world into the Harry Potter universe. Maybe she becomes a student at Hogwarts and uses her special skills to outwit the dark forces there, like Voldemort. She could also form new friendships with the main characters, and her different perspective on magic might change the way things happen in the story.
The plot might involve Harry using his magic in an unorthodox way to summon Cthulhu. Maybe he stumbles upon an ancient spell in the restricted section of the library. Once summoned, Cthulhu could bring chaos to the wizarding world. The dark and eldritch powers of Cthulhu would be in stark contrast to the more traditional magic in Harry Potter. This could lead to a struggle between the forces of Hogwarts and the indescribable horror of Cthulhu.
It could be really exciting. You might wish to be a powerful vampire right from the start. Maybe have the ability to compel anyone easily. And you could use your wishes to change the fates of some characters, like saving those who die in the original story.
If an OC was reborn in Vampire Diaries fanfiction, they might bring a fresh perspective. They could start off as a normal human and gradually get drawn into the supernatural world. They might befriend Elena and her friends, or they could end up on the wrong side of Klaus or other villains. It would all depend on how the writer develops the character and the story.
Another interesting aspect is how the different worlds' rules are bent or combined. For instance, the concept of a Horcrux in 'Harry Potter' and the way vampires can be killed in 'Vampire Diaries'. In some fanfictions, these two ideas are merged. Maybe a vampire tries to use a Horcrux - like mechanism to avoid being killed, or a wizard has to figure out how to destroy a vampire in a way similar to destroying a Horcrux. This creates a whole new set of challenges and mysteries for the characters to solve.
In 'harry potter is a demigod fanfiction', we could see a lot of changes to the existing story. For example, the magical world might have to interact with the world of the gods. There could be new enemies, like monsters from Greek or Roman mythology. Also, Harry's relationships with his friends and teachers might be different. His identity as a demigod could make him both more powerful and more of a target. He might have to go on quests like a typical demigod, which would be very different from his normal Hogwarts adventures.
It could entail a story about a Nundu being used in the Triwizard Tournament as an unexpected and extremely dangerous challenge. The champions would have to figure out a way to survive the encounter with the Nundu without using dark magic.
There could be major power shifts. For example, the main characters might start to rule Hogwarts in an 'Overlord' - like way instead of following the traditional school hierarchy. Maybe Harry Potter himself could become more of a tyrant - like figure, using his powers not just for good but also for self - gain.
I really like 'A Magical Merge'. In this fanfic, the Hogwarts students find themselves in Mystic Falls for some reason. It plays with the idea of how the wizards would react to vampires and werewolves. There are some great character development arcs, especially for characters like Elena who has to adapt to this new world of magic wands and spells. It also has some really cool fight scenes between the vampires and the dark wizards.