One character that would be great to add is a time - traveling adventurer. This character could accidentally bring Billy and Mandy to different time periods. They could end up in the prehistoric era, where they meet dinosaurs and cavemen. Or they could be in the future, in a world full of high - tech gadgets that they don't understand. The time - traveling adventurer could then be their guide through these strange times, while also having his own motives and secrets.
Hermione Granger from the 'Harry Potter' series could be an interesting addition. She could use her magic knowledge to help the Ghostbusters understand the more esoteric aspects of the ghosts they're dealing with. Maybe she could even develop spells that work in tandem with the Ghostbusters' technology.
A possible plot for Billy and Mandy crossover fanfiction is that they end up in a world of magic school. Billy is completely clueless about magic at first and keeps causing chaos in the classes. Mandy, on the other hand, is intrigued by the different types of magic and tries to learn as much as she can. They get involved in a competition between different houses in the school, and their friendship is tested as they face various challenges and temptations during the competition.
Granny from the series could be a great addition. She could act as a sort of wise advisor to Mandy and Billy, providing them with valuable advice based on her own experiences. Her no - nonsense attitude could also add some humor to the story, especially when she scolds Billy for his silly mistakes or when she encourages Mandy to be more open - hearted.
Lex Luthor would be an obvious choice. His complex relationship with Superman could add a lot of drama. Also, Supergirl could make an appearance. She could team up with Superman to fight a common enemy while also bonding with Lois as another strong female character.
In a crossover with 'Star Wars', Luke Skywalker could be a great addition. He could teach the boys about the Force and they could go on a mini adventure together. Also, Princess Leia could be added. She could bring her leadership and strong - willed nature to the group. They could all work together to fight off some Imperial threats that somehow end up in their world.
In a crossover with 'Final Fantasy', Cloud Strife could be added. He is a skilled warrior with a cool sword. He could join Spyro in his adventures and offer his combat skills against powerful enemies. Their different fighting styles, Spyro's magic and flight and Cloud's swordsmanship, would make for an interesting dynamic.
The Doctor from 'Doctor Who' would also be a great addition. With his time - traveling TARDIS, he could bring the characters of Journey to the West to different time periods and planets. He could also help them with his vast knowledge and advanced technology when they face difficult situations, like when they need to break a powerful spell that is beyond their normal capabilities.
Naruto himself is often a popular choice. His energetic and determined personality can bring a lot of action and positive energy to the fanfiction. Billy is also quite popular as his naivete and simple - mindedness can create humorous situations when paired with Naruto's more complex ninja world. And Mandy, with her dark and controlling nature, can be an interesting contrast to Naruto's heroic ideals, making for some great character - driven stories.
Billy and Mandy themselves are obviously very popular. Fans love to write about their adventures and how they interact with each other. Grim is also a key character that often features prominently. His role as a sort of guide or mediator in their crazy world is always interesting to explore in fanfiction.
A young journalist could be added. He's eager to make a name for himself by getting the inside scoop on Billy the Kid. He follows Billy around, trying to interview him and in the process, gets caught up in the dangerous world of the outlaws.