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.
Naruto could fit in as a new kind of hero. He'd bring his own set of powers like chakra control and jutsu casting. He might start as a stranger in the Ultimate Spider - Man world and then become an ally to Spider - Man.
He could fit in as a mentor figure. His years of experience as a hero would be valuable to the younger members. He could train them in combat and strategy.
One common theme is teamwork. Since both Spider - Man and the Fantastic Four are superheroes, fanfictions often explore how they can combine their powers and skills to overcome challenges. For example, Spider - Man's web - slinging and acrobatics can be used in tandem with the Fantastic Four's unique abilities like Mr. Fantastic's stretching or the Invisible Woman's invisibility shields.
He could fit in as a mentor figure. His experience dealing with super - powered threats in his own world would be valuable. He could train the newer Avengers in hand - to - hand combat or how to handle situations on the street level.
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, if Spider - Man joins the Fantastic Four, there could be some really cool team - up scenarios. They might face new and more powerful villains together. For example, they could take on a super - intelligent alien that requires both Spider - Man's agility and the Fantastic Four's unique powers to defeat.
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.
One interesting crossover could be Spider - Man and The Fantastic Four teaming up to fight a powerful new villain that neither could defeat alone. Maybe a villain with the ability to manipulate time and space, forcing them to combine their unique powers.
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.
Spider - Man could fit in as the tech - savvy member. His knowledge of creating his own web - shooters and other gadgets could be useful for the Justice League. He could upgrade their equipment or come up with new ways to fight villains.