Well, you could start by having a common villain. For example, the Green Goblin could team up with a mutant - hating group. This would force Spiderman and the X - Men to work together. Spiderman might be initially against the X - Men because he's not used to their way of doing things, but as they face the common threat, they start to understand each other.
You could also do a story where Peter Parker is a student at Xavier's school. He's trying to keep his identity as Spiderman a secret from the other mutants. But then, something happens that forces him to reveal his powers. This could create a whole new dynamic within the school, with some mutants being jealous of his abilities and others wanting to help him train and use his powers more effectively.
A different approach would be to have a time - travel element. Spiderman could be sent back in time to the early days of the X - Men. He could help them in some of their first battles against anti - mutant forces. In return, the X - Men could give him some knowledge or technology that he can use in his own time to fight crime better. This would create an interesting connection between the two universes and allow for a lot of character development.
In a more complex combination, Wonder Woman's connection to the gods could give Spiderman a temporary boost in his powers. Maybe she calls on a god to enhance his spider - sense so that it can detect threats from other dimensions, which would be useful when they face a villain who has the ability to open portals to different realms.
A great way to combine them is through a shared technology. Maybe some of the tech from the Omnitrix gets mixed up with Oscorp technology. This creates new villains or new powers for our heroes. They then have to figure out how to deal with these new situations. It could also lead to some interesting character development as they learn from each other. For example, Ben could teach Spider - Man about the different alien cultures he has encountered through his travels, and Spider - Man could show Ben how to better handle the public and keep his identity a secret in a big city.
In a fanfic, the magical and Pokemon worlds could merge due to a spell gone wrong. Alain could befriend some of the Hogwarts students and introduce them to Pokemon. The Hogwarts grounds could become a new place for Pokemon training, with spells and Pokemon abilities interacting in interesting ways. For instance, a Pokemon's electric attack might interact with a Lumos spell in an unexpected way.
Well, one way could be in a crossover fanfic. Imagine a world where the mutants from X - Men, the powerful Superman, and for some strange reason, lemons are all part of the same story. Maybe the lemons could have some sort of special power in this fanfic universe, like they can heal mutants or weaken Superman for a short time. It's all about creativity in fanfic writing.
Well, sometimes the comics might show a multiverse where Spiderman stumbles into a world dominated by dinosaurs. Or it could be a villain's scheme to use dinosaur DNA to create powerful monsters that Spiderman has to stop.
One idea could be that Spiderman gets caught in the middle of a conflict between the X - Men and one of their powerful enemies. Maybe Magneto is causing chaos in New York and Spiderman tries to stop him on his own at first, but then realizes he needs the X - Men's help. The X - Men, in turn, are initially skeptical of Spiderman but come to respect his skills.
Spiderman's strength and Silk's ability to create strong webs could be combined. For example, Silk could create a web - net and Spiderman could use his strength to throw it at a large group of enemies, trapping them all at once.
Well, it could start with a scientific experiment gone wrong that creates portals between Spiderman's world and the world of the monster girls. Spiderman gets sucked into this new world and has to find a way back. Along the way, he befriends some of the monster girls who help him on his journey.
Well, sometimes they're brought together in unexpected storylines. Like, Spiderman might encounter a supernatural threat while teaming up with Deadpool to save the day.
The characters' abilities can be combined in various ways. For example, a Naruto character could use their chakra to enhance the physical strength of a Resident Evil character. So, if Leon Kennedy was given some chakra by a Naruto ninja, he could become even more powerful in combat. Also, a Resident Evil character's knowledge of weapons and survival skills could be used to help Naruto characters in the Resident Evil world. For instance, Ada Wong could teach Naruto characters how to use guns more effectively. In terms of the harem, a female character's charm or intelligence from either world could be used to influence the group dynamics. A Resident Evil girl could use her scientific knowledge to help Naruto characters understand the viruses better, and in return, they could teach her about chakra.