One possible scenario could be that Quicksilver uses his super - speed to outmaneuver Ultron's attacks in the crossover. Maybe he teams up with both the X - Men and the Avengers, bringing his unique skills to the fight. For example, he could quickly disrupt Ultron's robotic minions or even steal important components from Ultron himself while the other heroes engage in direct combat.
There's a fanfiction where Storm's weather - controlling abilities are crucial in an Avengers - level threat. The X - Men and Avengers have to work together to stop a global weather - based disaster. In some crossovers, Cyclops' optic blasts are combined with Hawkeye's archery skills in a strategic way. They create a unique combat style that helps the combined team take on enemies. Also, Rogue's power - absorbing ability can lead to some really interesting interactions when she absorbs the powers of various Avengers members.
A genetic experiment gone wrong could also be a great plot for the crossover. A mad scientist tries to combine mutant and super - soldier DNA. This creates a new breed of dangerous creatures. The X - Men, with their knowledge of mutants, and the Avengers, with their combat skills and high - tech resources, must work together to contain the threat. Jean Grey's telepathic powers could be crucial in finding the source of the experiment, while Iron Man uses his technology to develop a way to reverse the genetic changes.
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 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.
Storm and Thor would be an amazing pairing. Storm has control over the weather, and Thor is the God of Thunder. They could create some truly epic weather - related attacks together. Imagine them combining their powers to create a massive super - storm to take down a powerful enemy. Also, Jean Grey and Doctor Strange would be very interesting. Jean Grey's powerful psychic abilities and Doctor Strange's magic could lead to some really mind - boggling interactions. They could explore different dimensions of the mind and magic together, perhaps even finding new ways to use their powers in combination.
Wolverine and Captain America would be a really interesting pairing. Wolverine's rough - and - tumble, no - nonsense attitude combined with Captain America's moral leadership could create some great interactions. They could also learn from each other's combat styles, with Cap teaching Wolverine some more strategic moves and Wolverine showing Cap some of his dirty - fighting techniques.
I think Wolverine and Black Widow would be an interesting pairing. Wolverine's rough and tough nature combined with his regenerative abilities, and Black Widow's spy skills and combat expertise could create some really cool action scenes. They could also have some great banter as they come from very different backgrounds.