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.
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.
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.
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.
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.
Sherlock Holmes could be an interesting addition. His deductive skills would complement the spy work of the U.N.C.L.E. agents. He could help them solve complex mysteries and outwit their enemies with his brilliant mind.
Luffy from One Piece could also be a great addition. His carefree yet strong - willed nature would create some fun dynamics. He might be amazed by the Air Gear skating and try to incorporate it into his own fighting style on the Grand Line. Plus, his rubber body could lead to some really unique skating stunts.
In a crossover with the Mortal Kombat universe, characters like Scorpion or Sub - Zero could be added. Their unique fighting styles and powers would add a new dimension to the Avengers' battles. Another character could be Goku from Dragon Ball Z. His super - strength and ability to fly and use energy blasts would make for some exciting interactions with the Avengers.
Well, if it's a crossover with 'Star Wars', characters like Luke Skywalker could be added. He could use his Jedi skills to help the 'Race to Witch Mountain' characters in their journey. His knowledge of the Force might be useful against the enemies they face.
A character with the power of wind could be added. This character could help Sharkboy and Lavagirl in their adventures by providing fast transportation or creating powerful gusts to fend off enemies. For example, when they are facing a horde of flying creatures, the wind - powered character could blow them away.