Another possibility is a character exchange. Maybe Elsa could lose her powers and in her search for a solution, she ends up in the world of The Little Mermaid. There, she meets Ariel who decides to help her. Ariel could introduce Elsa to the magic of the sea, which might have some connection to Elsa regaining her powers. Meanwhile, one of Ariel's sisters could visit Arendelle and learn about the different kind of magic there.
In fanfiction, a common way to merge the two worlds could be through a common enemy. A dark force could be threatening both Arendelle and the underwater kingdom. So, characters from both stories would have to team up. Kristoff and Sven could join forces with Flounder and Sebastian. They would bring their unique skills - Kristoff's survival skills in the cold, Sven's strength, Flounder's knowledge of the sea, and Sebastian's musical charm - to fight against this common threat.
One way they could merge is through a magical event. For example, a powerful sorcerer in the Frozen universe could accidentally open a portal to the underwater world of The Little Mermaid. This would lead to characters from both stories interacting. Anna and Elsa might be curious about this new underwater world and its inhabitants, while Ariel could be fascinated by the ice and snow in Arendelle.
Typically, they use some sort of magical or dimensional rift to merge the two worlds. For example, Naruto could be sucked into a portal that leads him to the High School DxD universe. Or, it could be a more complex plot device like a powerful villain trying to combine the two worlds for their own gain.
Another way is through a powerful artifact. Suppose there is an ancient artifact in Highschool DxD that has the power to connect different dimensions. When it is activated, it links to the world of Overlord. This could lead to all sorts of storylines, like characters from both sides trying to control the artifact for their own purposes, or working together to keep it from falling into the wrong hands.
Authors often start by finding a connection point. For example, a magical portal could be the link between the Harry Potter world and Sesshomaru's world. This allows for the characters to meet in a somewhat believable way.
One interesting fanfiction could be about Elsa from 'Frozen' meeting Ariel from 'The Little Mermaid'. Maybe Elsa uses her powers to create a magical ice path in the ocean for Ariel to swim on. They could exchange stories about their different worlds, Elsa about her snow - covered kingdom and Ariel about her underwater home.
In fanfiction, the merge could happen because of a powerful jutsu gone wrong. This jutsu could distort space - time and connect the two universes. In the merged world, the Gundam pilots would have to deal with the threat of ninja attacks. They could use their Gundam building skills to create mechas that are better equipped to handle ninja abilities like genjutsu. Meanwhile, the Naruto characters would be fascinated by the Gundam battles and might try to participate using their own combat skills.
In a Naruto and Batman crossover fanfiction, their worlds could merge in a few ways. One way could be through a portal. Maybe some sort of evil magic or advanced technology creates a portal between Konoha and Gotham. This would bring Naruto and his friends into Batman's world, or vice versa. Another possibility is that a powerful villain has the ability to manipulate space - time and decides to bring the two worlds together for his own evil purposes.
Their worlds could merge in a lot of ways. For instance, the technology from the Avengers' world could be introduced to Lilo and Stitch's Hawaiian setting. Maybe Stark Industries could set up a research facility in Hawaii to study Stitch's biology. And in return, Lilo could introduce the Avengers to the local culture and the concept of 'ohana', which could strengthen the Avengers' sense of family within the team.
Well, it could start with a common enemy. There could be an entity that is a threat to both Inuyasha's world and Riddick's. This would force them to team up. For example, a powerful demon in Inuyasha's world has some sort of connection to a dangerous alien race in Riddick's universe. They would have to combine their combat skills, with Inuyasha's swordsmanship and Riddick's hand - to - hand combat abilities.
The main characters are often Elsa and Ariel. Elsa has her ice powers which create an interesting contrast with Ariel's life in the sea. They are both strong female characters with different abilities and backgrounds, so it's exciting to see them interact in fanfiction.
The merge can also happen on a more conceptual level. For example, the idea of friendship and teamwork, which are important in both series, could be the connection. Naruto's belief in his friends and the Digidestined's bond with their Digimon could be seen as similar. This similarity could be the starting point for the two worlds to interact in fanfiction. Writers might have Naruto and the Digidestined team up to fight a common enemy that threatens both the Digital World and the ninja world. They would combine their unique powers and skills to achieve victory.