Another interesting concept could be a crossover of characters. Picture Hermione Granger meeting Bella Swan. Hermione could introduce Bella to the world of magic and how knowledge and intelligence can be as powerful as any vampire or werewolf ability. They could team up to solve a mystery that involves both the magical and the supernatural elements of their respective worlds.
A third idea is to explore the idea of a magical creature from Harry Potter, like a centaur, existing in the Twilight world. The centaur could have knowledge about the vampires and werewolves that the other characters don't. This could lead to a whole new adventure where the characters from both universes have to work together to understand and deal with this new discovery.
A more complex idea would be a plot where Voldemort somehow discovers the immortality of the vampires in the Twilight world. He tries to make an alliance with them to gain more power, but the vampires are hesitant as they don't want to get involved in the wizarding war. This would create a lot of political intrigue and power struggles.
One idea could be a wizard from the Harry Potter world accidentally traveling to Forks. Maybe they mistake the vampires in Twilight for some kind of dark magic creatures. They could then try to use their magic to either fight or understand them.
One idea could be a spell gone wrong in the wizarding world that somehow transports some Hogwarts students to Forks. They then have to deal with the vampire and werewolf situation there while also trying to find a way back home. Maybe Hermione could be intrigued by the science - like aspects of the Twilight world that seem so different from magic.
One idea could be a story where a wizard from the Harry Potter world accidentally travels to the Twilight universe. Maybe he gets caught up in the vampire - werewolf conflict there while trying to find a way back home. He could use his magic in unexpected ways, like creating magical wards to protect the Cullens' home in a different way than they're used to.
Well, you could have Hogwarts located near Forks. So, the students at Hogwarts might sometimes interact with the vampires and werewolves from Twilight. For instance, there could be a forbidden forest area that connects the two worlds in a way, and strange things start to happen when characters cross over.
Another interesting concept is a character from Harry Potter, like Harry himself, ending up in Forks. He could discover the existence of vampires and werewolves there. He might try to use his magic secretly while also being fascinated by the different supernatural rules in that world. He could even form an unlikely friendship with Bella or Edward, sharing his knowledge of magic with them.
One idea could be having Elena from 'The Vampire Diaries' meet Bella from 'Twilight'. Maybe they discover that they have more in common than they thought, like their love for vampires who are trying to resist their more primal instincts. They could team up to help their vampire boyfriends deal with some common threats in both their worlds.
One idea could be that the vampires in Twilight are somehow affected by the magic in the Harry Potter world. Maybe they can't enter Hogwarts because of the powerful protective spells. Another could be a character from Twilight attending Hogwarts and having to deal with both the wizarding world's rules and their vampire nature.
One idea could be that the vampires in Twilight are discovered by the wizarding world in Harry Potter. The wizards might be intrigued and cautious at the same time. They could study the vampires' immortality and special abilities, while the vampires might be interested in the magic that wizards possess.
One idea could be a crossover with the Lord of the Rings. Imagine Harry and his friends traveling to Middle - earth to help in the fight against Sauron. They could use their magic in new and unexpected ways in that different magical realm.