The Thing might be the one. He has a bit of a short temper sometimes. If Spider - Man accidentally gets in his way or says something that rubs him the wrong way, he could start a conflict.
On the other hand, Spider - Man himself could unknowingly start a conflict. His quick - witted and sarcastic remarks might be misinterpreted by the Fantastic Four as disrespect. For instance, if he makes a joke about Mr. Fantastic's stretching abilities in the wrong context, it could lead to some tension between them.
Perhaps Spider - Man would be initially distrusted by the Fantastic Four. He is a bit of a lone wolf compared to their more family - like dynamic. But then they would realize his good intentions and start to work together. For example, he could use his web - slinging to help Mr. Fantastic reach places during a rescue mission, while the Invisible Woman could provide cover for Spider - Man when he's outnumbered.
Obviously, Spider - Man himself is very popular. His quick - wittedness and acrobatic fighting style make him a fan favorite in any story. He can bring a sense of humor and energy to crossovers.
Sue Storm could be a great character to focus on. In a crossover, she could use her force fields in new and creative ways. Maybe she has to protect Spider - Man from some of his more dangerous enemies when he's outnumbered. Also, her relationship with Reed Richards could be explored further in the context of a crossover. How would she react to Spider - Man's more impulsive nature? And how would she use her powers to help Spider - Man and the Fantastic Four work together more effectively? There are a lot of interesting storylines that could be developed around her.
In fanfiction, Spider - Man could fit into the Fantastic Four by bringing in his own set of values. He has a strong sense of responsibility which could influence the group. Also, his intelligence in the field of science (he's no slouch in that department) could be an asset when the Fantastic Four are dealing with scientific problems. He might also introduce new ways of using their powers based on his own experiences in combat.
Flash could also be a likely candidate. He's quick - tempered at times. If he sees Naruto doing something that he thinks is wrong or dangerous, like using his ninjutsu in a populated area, he might immediately confront Naruto and this could spark a conflict. Also, Naruto's headstrong nature means he won't back down easily if he feels he's being wrongly accused, so it could escalate quickly.
Batman and Superman are likely candidates. Their different ideologies often put them at odds. Batman is more of a cautious and sometimes cynical character, while Superman is an idealist. In a fanfiction, Batman might suspect Superman of becoming too powerful and a potential threat to humanity, which could start a conflict that divides the Justice League.
Voldemort could be a great villain. Imagine him getting his hands on some of Spider - Man's technology. He could use it to enhance his Death Eaters' powers or even create new, more powerful dark magic. Spider - Man and Harry would have to put aside their differences in fighting styles and worlds to stop him.
The Green Goblin could be a likely villain. His chaotic nature and powerful glider - based attacks would pose a great threat to both Wonder Woman and Spider - Man. He could be after some sort of powerful artifact that Wonder Woman has knowledge of, and Spider - Man gets involved trying to stop him from getting it.
Definitely Sans and Spider - Man. Sans is known for his laid - back yet intelligent nature. He would be curious about Spider - Man's powers and his sense of justice. Spider - Man, on the other hand, would be intrigued by Sans' magic and his non -chalant attitude towards danger. Their interactions could range from friendly banter to serious discussions about how to protect their respective worlds.
Loki could be the one most likely to start a conflict. He's always been a bit of a troublemaker in the Avengers world. In a meeting with Percy Jackson and his group, Loki might see an opportunity to cause chaos. He could try to manipulate the situation by pitting the Avengers against Percy and his friends, thinking he can use the confusion to his advantage.