Well, the Avengers could be shrunk down to Pokemon - size through some strange scientific experiment gone wrong. In the world of Pokemon Go, they would then have to adapt to being so small and deal with the challenges that Pokemon face. They could team up with a group of Pokemon trainers who are initially shocked to see these tiny versions of the Avengers. Together, they could fight against an evil organization in the Pokemon world that wants to use the Avengers' shrunken state for their own evil plans.
In a Pokemon Go Avengers fanfiction, perhaps a new Pokemon is discovered that has powers similar to the Infinity Stones. This attracts the attention of both the Pokemon Go community and the Avengers. The Avengers see it as a potential new threat or power source. The Pokemon trainers, on the other hand, are just excited to catch a new and powerful Pokemon. They end up having to work together. The Avengers might learn about the unique bond between trainers and Pokemon and use that knowledge in their battles.
One way to combine them could be through a crossover device. For example, a powerful Pokemon like Mewtwo could accidentally open a portal to the Avengers' universe. Then, the Avengers might mistake the Pokemon for some kind of new alien threat. They would try to contain it, but soon realize that the Pokemon has no malicious intent. Meanwhile, the Pokemon trainer who follows Mewtwo through the portal has to team up with the Avengers to find a way back home and also deal with any villains in the Avengers' world that might try to use the Pokemon for their own gain.
One way is through character crossovers. For example, a Pokemon trainer could end up in the Fairy Tail world and use their Pokemon to help in quests. They might use a Water - type Pokemon like Squirtle to put out fires during a mission. Another way is to combine the magic systems. Maybe Fairy Tail magic could be used to enhance a Pokemon's abilities, like using Fire Dragon Slayer magic on a Charmander to make its flames even hotter.
One way is to create a character who is a super genius and has a connection to both the Harry Potter and Avengers universes. For example, this character could be a wizard who discovers a way to travel between dimensions and ends up in the Avengers' world. They could use their magic to help the Avengers in unique ways, like using spells to enhance the heroes' powers or to heal them quickly.
One way to combine the My Hero Academia and Avengers universes in a fanfic could be through a dimension - hopping plot device. Maybe a character from My Hero Academia accidentally discovers a portal to the Avengers' universe. For example, Deku could find himself in New York City and meet Iron Man. Iron Man, being the genius he is, would be intrigued by Deku's Quirk and want to study it. This could lead to an exchange of ideas and technology between the two universes, like Deku learning about Stark technology and using it to enhance his hero suit in his own universe.
Thor could also be very powerful. He has his Asgardian strength and his hammer (or Stormbreaker). If he were to also gain the ability to use magic like the wizards in Harry Potter, he could be near - invincible. He could call lightning while also casting spells. And his knowledge of different realms could give him an edge in any battles that span across the combined universes.
Imagine that the Avengers' tower is transported into the Pokemon Go world. The Avengers are confused and have to figure out how to get back to their own universe. Meanwhile, the local Pokemon are both curious and a bit scared of these strange new beings. The Avengers start to learn about Pokemon battles and how to catch Pokemon. They form alliances with some of the more intelligent Pokemon species and together they face off against a group of villains who want to use the Avengers' technology to take over the Pokemon Go world.
Another aspect could be the fusion of their worlds' elements. In Mortal Kombat, there are dark and hellish realms, while Naruto has its own unique elemental chakra system. A fanfiction could explore what would happen if the dark magic from Mortal Kombat's realms seeped into the Naruto universe and affected the chakra. Maybe it could create new types of jutsu or even corrupt some of the ninja. It would be a great source of conflict and adventure in the story.
Maybe a character from the Naruto universe, like Naruto himself, could be summoned to Wakanda due to some powerful magic or technology gone wrong. He would bring his ninja skills and values, and his presence could disrupt the power balance in Wakanda. This would lead to various interesting scenarios, such as Naruto learning about Wakandan culture and the Wakandans learning about ninja techniques.
Well, it could be that the magic from Harry Potter's world and Android 18's advanced technology somehow interact. Maybe the spells in Harry Potter are used to enhance or modify Android 18's abilities. For example, a spell could be cast on her to make her energy blasts more powerful or her speed even faster.
One way is through the power systems. The chakra in Naruto and the energy used in Street Fighter can be merged. For instance, a character could use chakra to enhance the power of a Street Fighter move like Chun - Li's Spinning Bird Kick.
One way is through a portal. For example, a portal could open up in Ben's world that leads to the world of Percy Jackson. Ben, being the curious and adventurous type, steps through and finds himself in a world full of gods, demigods, and monsters. Another way could be a magical spell gone wrong. A character in Percy's world tries to cast a spell for something simple but it ends up pulling in Ben and his Omnitrix.