One idea could be Deadpool trying to join the X - Men but constantly messing things up with his unorthodox methods. For example, he might start a fight with the wrong mutant during a training session because he misread the situation.
A third plot could be that an old enemy of Deadpool creates a device that can control mutants. The X - Men are affected, but Deadpool, due to his unique physiology, is immune. He has to find a way to break the control over the X - Men. Along the way, he discovers that some of the X - Men have secrets that they were hiding even from each other, and Deadpool has to decide whether to expose these secrets or use them to his advantage to save the day.
Another plot could involve Deadpool getting a mission from a mysterious source that seems to be related to an X - Men threat. He then has to team up with the X - Men, reluctantly at first. During the mission, his unique sense of humor clashes with the more serious X - Men like Cyclops, creating lots of comical and tense moments. Deadpool could also introduce some wild and crazy tactics that the X - Men have never considered before, which might actually end up helping them in the end.
One idea could be Deadpool trying to join the X - Men but constantly causing chaos with his unorthodox methods. He might start off by 'auditioning' in the most over - the - top way, like showing up with a bunch of his chimichanga - loving friends. Another could be Deadpool getting involved in an X - Men mission where his fourth - wall - breaking abilities actually help them in unexpected ways, like when they're facing a mind - reading villain and he can just tell the reader what the villain is thinking.
One idea could be a battle against a common enemy that is too powerful for either the Avengers or the X - Men alone. For example, a powerful alien race that wants to enslave mutants and humans alike. The two groups would have to combine their unique powers and skills to defeat this enemy. Another plot could involve a misunderstanding between the two teams that leads to a brief skirmish before they realize they are on the same side. For instance, the Avengers might mistake the X - Men's actions as a threat to humanity when in fact they are trying to protect a mutant from a rogue government agency.
One idea could be that a powerful cosmic entity threatens both the Avengers' world and the X - Men's. So they have to combine their forces. For example, Iron Man could work with Beast to develop new tech to counter the entity's energy. Another could be a mutant - hating villain who uses some of the Avengers' tech to target mutants, forcing the two groups to team up to stop him.
One idea could be that a powerful mutant villain emerges who is able to manipulate time. This causes disruptions in both the X - Men and Avengers' universes. The X - Men, with their knowledge of mutants, and the Avengers, with their vast resources and superpowers, have to team up to stop this villain. They start off distrusting each other, especially because the Avengers are sometimes seen as anti - mutant by some in the X - Men world. But as they face challenges together, they learn to work as one.
A great 'X Men Crossover Fanfic' idea could involve crossing over with the Buffy the Vampire Slayer universe. The X - Men could help Buffy and her gang fight some really powerful vampires that are immune to normal slayer weapons. Another idea is an X - Men and Doctor Who crossover. The Doctor could land in the X - Men's world and they could team up to stop some time - traveling villain who is trying to change mutant history. There could also be a crossover with the Ghostbusters. The X - Men and the Ghostbusters could work together to stop some mutant - created ghosts that are causing chaos in New York.
Another plot could involve Deadpool getting a contract to 'kidnap' Wolverine for some mysterious client. But when he tries, he realizes there's more to Wolverine than he thought. They end up on a journey together to figure out who the client is and why they want Wolverine. Along the way, they share their past traumas and form an unlikely bond.
One idea could be that Loki and Deadpool are forced to team up by some cosmic entity. Loki is initially annoyed by Deadpool's constant chatter and antics, but gradually comes to respect his unorthodox fighting style. Another plot could involve Deadpool getting transported to Asgard and causing chaos, with Loki having to either clean up his mess or use it to his advantage. Maybe they could also be on a quest to retrieve a powerful artifact that has been stolen and is being misused in the multiverse.
A cool plot could be that a new, extremely powerful mutant emerges who has the ability to control all forms of energy. This mutant's powers are so great that they start to disrupt the fabric of reality. The Avengers and the X - Men both realize that this mutant needs to be controlled or taught to manage their powers. However, different factions within both groups have different ideas on how to approach the situation. Some want to be more forceful, while others believe in a more diplomatic approach. This leads to internal conflicts within the combined group as they also battle external threats caused by the mutant's out - of - control powers.
One idea could be that Superman is somehow transported to the X - Men's universe. He teams up with the X - Men to fight a common enemy like a powerful mutant who has the ability to nullify Superman's powers on a large scale. They have to figure out how to combine their skills and powers to overcome this threat.
One idea could be that the kryptonite in Smallville starts to affect the mutants in strange ways. Maybe it suppresses or enhances their powers in unexpected manners. For example, Wolverine's healing factor could be slowed down by kryptonite exposure, leading to a real threat when he gets injured. Another plot could involve Lex Luthor getting his hands on some mutant DNA and trying to create his own super - soldiers, which would draw the attention of the X - Men. They could team up with Clark Kent to stop Luthor's evil plans.