The Volturi could be a main antagonist. They might see the intrusion of the wizarding world into their territory as a threat to their power and secrecy. They could try to eliminate Harry Potter and Charlie Swan to protect their vampire - centric world in Forks.
Perhaps there's a spell gone wrong that links the Twilight world and the Harry Potter world. Charlie Swan and Harry Potter find themselves in a situation where they have to reverse the spell. They encounter various characters from both universes. Charlie has to learn about the wizarding world quickly, and Harry has to deal with the strange vampire and werewolf dynamics in Forks. They face challenges like dealing with the Volturi who see the magical intrusion as a threat, and at the same time, they have to find a way to send everyone back to their respective worlds.
Yes, there are some out there. You can start by looking on fanfiction websites like Archive of Our Own. Some fanfictions might explore the unique relationship between a character from Harry Potter and Charlie from Twilight, perhaps in a crossover adventure where they team up to face a common threat or solve a mystery that spans both their magical worlds.
One common theme could be the clash of different magical systems. Harry Potter has its own set of spells and magic, while Twilight has vampires and werewolves with their own abilities. Another theme might be friendship or alliance building between the two worlds. Maybe they have to work together to fight a greater evil.
There could be a story where Charlie Swan adopts a magical creature that has escaped from the wizarding world. He doesn't know it's magical at first but then realizes it when strange things start to happen around his house. He then has to figure out how to return the creature to its proper place in the wizarding world without getting into trouble with the Ministry of Magic.
Charlie Swan can be integrated as a wizard who has chosen to live in the Muggle world. He could have his own set of magical abilities that are useful in both worlds. For example, he might be really good at spells that can calm wild animals, which would be useful in his job as a sheriff in Forks and also in the wizarding world when dealing with magical beasts.
One great 'Harry and Charlie Swan Fanfiction' is 'The Unlikely Bond'. It beautifully weaves their characters together in an alternate universe where they team up to solve a mystery in Forks. The story has a great balance of character development and plot twists.
One interesting fanfiction could be about Charlie Swan's perspective during Bella's transformation. It might explore his internal struggle of seeing his daughter turn into a vampire. Another could be a story where Charlie discovers the world of the supernatural much earlier and has to deal with it secretly. And there could also be a fanfic that focuses on Charlie's past and how it influenced his relationship with Bella.
One interesting 'Harry Potter Charlie Swan' fanfiction could be one where Charlie Swan discovers the wizarding world of Harry Potter. Maybe he stumbles upon a magical creature in the forests near Forks and that leads him to Hogwarts. It could be a story full of adventure as he tries to understand this new world while still maintaining his connection to his life in Forks.
Another place to check could be some dedicated Harry Potter or Twilight fan forums. Sometimes, fans share their own written fanfictions on these forums. You might have to dig a bit deeper, but it could be worth it. There could be some really unique and interesting 'Harry Potter Charlie Swan' fanfictions that haven't made it to the bigger platforms yet.