In many fanfics, it often involves some sort of powerful magical artifact. For example, a special enchanted mirror that can show different universes and, with the right spell, can be used to step into them. Another way could be through a time - turned - portal. Since time and universes are often intertwined in magical theories, a malfunctioning time - turner might accidentally open a gateway to an alternative universe.
One interesting possibility is a universe where Voldemort won. It would be a dark and desolate world. The remaining Order of the Phoenix members would be in hiding, and Hogwarts would likely be a place of indoctrination rather than learning. Another could be a universe where Harry was sorted into Slytherin. He might have had a very different set of friends and challenges, and perhaps a more complex relationship with Draco Malfoy.
In a fanfic, the coexistence could be due to a multiverse theory. The multiverse has branches that include the Harry Potter universe and the DC universe where the Justice League exists. A disruption in the multiverse causes these two universes to start overlapping. As a result, the heroes and wizards have to collaborate. They might face a threat that is using the chaos of the overlapping universes to gain power, like a dark wizard who teams up with a supervillain from the DC universe.
Another concept could be the interaction of different magical systems. Maybe the magic in the Harry Potter world combines with that of another universe, leading to new spells or ways of using magic. For instance, a spell from a different universe might be adapted to work in Hogwarts, creating a whole new set of magical possibilities.
Harry Potter himself is still very popular in these fanfics. After all, the whole concept often revolves around him in some way, whether it's his journey through different universes or his interactions with other versions of himself or different characters from other universes.
There could be. A good fanfic would probably focus on the unique aspects of each universe and find a way to blend them. For example, the magic system in Harry Potter and the Devil Fruit powers in One Piece. It might start with a magical accident that transports a character from one world to the other and then explore how they adapt and interact with the new environment.
In theory, it's possible. If we consider that there are magical libraries or places where different kinds of magical knowledge are stored. It's not a stretch to think that some magical being might collect all kinds of stories, including fanfic from other fictional worlds. So Harry Potter could potentially stumble upon Percy Jackson fanfic in such a place.
A 'Harry Potter Cheshire Cat' fanfic could blend the two universes by having the Cheshire Cat enter the wizarding world through a magical portal. Once in the world of Harry Potter, it could interact with the characters using its unique abilities. For example, it could use its disappearing act to steal food from the Hogwarts' kitchen without being noticed by the house - elves.
Yes, there are some. Some fanfics manage to blend the magic of Harry Potter and the ninja world of Naruto in really interesting ways in the context of a final battle. They might have characters from both series using their unique abilities side by side, creating epic fight scenes.
Yes, there are likely some fanfics out there that blend the Harry Potter and Jurassic Park universes. Some might have wizards visiting the dinosaur - filled Jurassic Park, or perhaps dinosaurs being brought into the wizarding world. However, the quality and nature of these fanfics can vary greatly.