One idea could be that the Shadowhunters from The Mortal Instruments have to deal with the presence of vampires from Twilight in their world. Maybe they initially mistake the Twilight vampires for demons and chaos ensues. The Clary from The Mortal Instruments could form an unexpected alliance with Bella from Twilight to figure out how to co - exist.
One idea could be a crossover with the Harry Potter universe. The Shadowhunters could visit Hogwarts and interact with the wizards. They might have to team up to fight a common dark magic threat that combines the magic from both worlds.
One idea could be creating an OC who is a half - angel, half - demon. This would bring a unique perspective to the story as they struggle with their dual nature in the world of the Mortal Instruments. They could be on a quest to find a way to balance their two halves and in the process, interact with the main characters in unexpected ways.
Perhaps a spell gone wrong in the Charmed universe causes a portal to open to the Twilight world. The Charmed Ones end up in Forks and meet the Cullens. They initially suspect the Cullens of being dark magic users but then realize they are vampires. Together they face a common enemy that is drawn to the combined magic and vampire allure in the area.
A really cool plot for this crossover fanfiction could be that there is a prophecy in both the Twilight and The Originals worlds. It predicts that a certain character from each universe must join forces to save both worlds from a great evil. This could be a dark force that is slowly consuming all supernatural beings. Bella Swan could be the key in the Twilight world, and Hope Mikaelson in The Originals world. As they meet and start to work together, they face many challenges due to the different beliefs and power systems in their respective worlds. Their friends and families from both sides also get involved, creating a complex web of relationships and power struggles.
One plot idea could be that the vampires from Twilight possess some of the superpowers from the Marvel universe. For example, Edward could have Wolverine's healing factor in addition to his vampire abilities. This would make him even more indestructible and add a new layer to his character.
A plot could involve a Twilight vampire falling in love with a shadowhunter. The vampire has to deal with the shadowhunter world's rules about the supernatural, and the shadowhunter has to come to terms with loving a vampire, which goes against their training. This could lead to a lot of internal and external conflicts as they try to be together.
Well, a cool plot could be a love triangle between a Twilight character, a Naruto character, and another from their respective worlds. Say Bella is torn between Edward and Sasuke. Sasuke's brooding nature could be seen as similar to Edward's, but his ninja skills and different moral code would add an interesting dynamic. Also, the clans in Naruto could have some sort of connection to the vampire covens in Twilight, like they share a long - lost ancestor who had both ninja and vampire - like abilities.
One plot idea could be that Bella Swan, from Twilight, gets lost in Gotham City. Batman discovers her and at first suspects she's part of some new vampire plot in his city. But as he gets to know her, he realizes she's just a lost girl with her own set of supernatural problems. Another idea is that the Cullens move to Gotham for some reason. Edward and Batman cross paths and Edward's mind - reading ability both intrigues and annoys Batman. They could end up working together to stop a new breed of super - vampires that are terrorizing the city.
One idea could be that Naruto is somehow transported to the Twilight universe. He might mistake the vampires for some sort of new enemy ninjas. And the vampires could be intrigued by his strange powers. For example, his shadow clone jutsu could really puzzle them.
Perhaps in a post - war scenario in both the 'Harry Potter' and 'Mortal Instruments' universes, Hermione and Draco decide to collaborate on a project to bridge the gap between the two magical communities. Along the way, they face opposition from traditionalists in both worlds who don't want the two communities to mix. Their relationship could develop from reluctant partners to something more as they fight for their cause. This would involve a lot of character growth, exploring new magical concepts, and dealing with the politics of both worlds.