One idea could be that Buffy and her friends are called to New Orleans because of some strange vampire activity that seems linked to the Originals. Maybe the First Evil from Buffy's world has some connection to an ancient threat in the Originals' world, and they have to team up to stop it.
A plot could involve a spell gone wrong that transports Buffy into the world of the Originals. There, she meets Klaus and his siblings. At first, they're suspicious of her, but they soon realize her slayer abilities could be useful in their fight against a new and powerful enemy that is immune to normal vampire weaknesses.
One idea could be a love triangle between a vampire from 'The Vampire Diaries' and two characters from 'The Originals'. For example, a young vampire falls in love with Klaus but also has a deep connection with Elijah. This could create a lot of tension and drama as the two brothers have their own complex relationship. Another plot could involve a powerful vampire from 'The Vampire Diaries' coming to New Orleans and challenging the power structure that the Originals have built. This would lead to battles and political intrigue within the supernatural community.
Maybe the Avengers need the help of the witches from 'The Originals' to reverse a spell that an enemy has cast on Earth. Klaus and Loki could have some interesting interactions as two complex anti - heroes. The witches could use their magic to enhance the Avengers' weapons or powers in unique ways.
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.
Perhaps a powerful enemy from Merlin's past resurfaces and is causing chaos in both his world and the world of the Originals. Merlin and the Mikaelson family have to work together to defeat this enemy. The Mikaelsons' cunning and physical strength combined with Merlin's magic could make for an epic battle. Along the way, there could be interesting character interactions, like Elijah being impressed by Merlin's wisdom and honor.
One idea could be that a powerful witch from 'The Originals' world accidentally opens a portal to the 'Supernatural' world. Sam and Dean Winchester are drawn through it and end up in New Orleans. There, they team up with the Mikaelson family to fight a common enemy, like an ancient and powerful demon that is causing chaos in both their worlds.
One idea could be to cross the characters with another popular supernatural show's characters. For example, bringing in some characters from 'Buffy the Vampire Slayer' and seeing how they interact with the originals. Maybe Buffy could team up with Klaus to fight a new, powerful enemy that threatens both their worlds.
One idea could be that the Charmed Ones and Buffy team up to fight a powerful demon that has the ability to move between their two worlds. They have to combine their unique magical powers and slaying skills to defeat it.
One idea could be that Buffy and the Bones team have to work together to solve a mystery involving a supernatural entity that leaves behind skeletal remains with strange markings. The Bones team's scientific expertise and Buffy's knowledge of the supernatural would create an interesting dynamic.
Well, there could be a story where a powerful dark magic object from the wizarding world is brought to Sunnydale. Buffy discovers it and realizes it has the power to disrupt the balance between good and evil in her world. She then travels to Hogwarts to seek help from Harry Potter. Along the way, she meets various magical creatures and has to navigate the wizarding world's politics. Once she meets Harry, they discover that there is a common enemy behind all this, someone who wants to use the chaos in both worlds for their own gain, and they must work together to stop this mastermind.
Well, perhaps there's a spell in the wizarding world that accidentally opens a portal to Sunnydale. Buffy stumbles through and finds herself in Hogwarts. The Hogwarts students are initially confused by her, but when they see her fighting skills against the dark arts, they start to work together. Harry could learn a lot from Buffy about facing the unknown fearlessly, and Buffy could gain knowledge about magic from the wizards.