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.
A plot could involve a time - traveling aspect within the 'the originals crossover fanfic'. The characters could be sent to different time periods in the past or future, and they have to figure out how to get back while also dealing with the changes in their relationships and power dynamics. For instance, Elijah might end up in a time when vampires were first created and has to adapt quickly to survive and find a way back to his own time.
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.
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 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 a crossover where characters from the Originals visit Hogwarts. They could interact with the students and teachers, and perhaps there could be a magical mystery that they all have to solve together. For example, a dark magic spell that is affecting both the wizarding world and the vampire/werewolf world in the Originals.
A cool concept would be to have Caroline from The Vampire Diaries go to New Orleans and get involved with the Mikaelson family. She could use her vampire strength and her knowledge from Mystic Falls to help them out in some way. Maybe she becomes friends with Hayley and they work together to keep the Mikaelson family safe from their enemies. Also, Damon could end up in New Orleans and cause some chaos among the Originals, as he is known for his unpredictable behavior.
One interesting plot could be a crossover where a Twilight character gets involved in an Original family feud in New Orleans. Maybe Bella, with her unique ability to resist vampire powers, somehow ends up in the middle of a power struggle between Klaus and his siblings.
Another option is a crossover with the Pokémon world. The Phantom Thieves could travel to a region in the Pokémon world and discover that someone is using Persona powers to make Pokémon act out of control. They would need to catch Pokémon themselves and use their combined powers of Personas and Pokémon to stop the bad guy. It would be interesting to see how they interact with Pokémon trainers and the whole Pokémon ecosystem.