It adds new elements. For example, by bringing in characters or concepts from other universes. This can change the dynamics of the relationships between the original Harry Potter characters. It might also introduce new magic systems that force the Hogwarts characters to adapt.
One idea could be a crossover with the Lord of the Rings universe. Harry and his friends could find themselves in Middle - earth, having to deal with Sauron's forces while also using their magic. Another could be a crossover with the Star Wars galaxy, where wizards and witches interact with Jedi and Sith. Maybe they could learn from each other's use of the Force and magic.
A possible crossover could involve the magic system of Harry Potter being combined with the Gem technology in Steven Universe. For example, Hermione could study how the Gems' powers work and try to find a scientific explanation for it. On the other hand, Pearl could be fascinated by the complex spells in the wizarding world. And perhaps Dumbledore and Garnet could have a deep conversation about leadership and protecting their respective worlds.
Tinkerbell could be a new kind of magical creature in the wizarding world. She might be discovered by Hagrid in the Forbidden Forest. Since she's so small, at first the wizards might mistake her for a really tiny pixie. But then they'd realize she has different powers, like being able to create light and fix things with her fairy dust.
It could be really exciting. Maybe Harry Potter characters could visit Gotham City. Harry could use his magic to help Batman fight crime in a way that normal gadgets can't. For example, he could use spells to disarm villains from a distance or create magical barriers to protect civilians.
When starting 'harry potter crossover universe fanfiction', think about the overall theme. If you're crossing it with the Hunger Games, maybe it's about survival in a different kind of world. Then, develop your characters. Consider how a character like Katniss would fit into the wizarding world or vice versa. Don't be afraid to break some rules, but also stay true to the essence of both universes. Start with a hook, like a mysterious event that brings the two universes together, and then build your story from there.
One popular type of Lily Potter fanfiction might focus on her time at Hogwarts before she met James. It could explore her friendships, her studies, and how she dealt with the challenges of being a young witch in a magical school. For example, a story could show her in her Potions class, working hard to master difficult spells and potions, and forming a study group with her closest friends.
Magic in the RWBY universe could be seen as a new form of energy. Spells like Lumos could be used to light up the dark areas filled with Grimm. Wizards might have to adapt their magic to work with Dust. For instance, a Fire - making spell could be combined with Fire Dust to create a more powerful blast.
If Harry Potter was in the Marvel Universe, he'd probably end up at Xavier's School for Gifted Youngsters. Think about it, a school full of mutants with special powers, and here comes Harry with his wizardry. He'd either be a great asset or cause a bit of a stir. His magical abilities would be a new kind of power that the X - Men and other mutants might not be used to dealing with.
One possibility could be a story where a character from Harry Potter becomes a servant in the Fate universe. For example, Harry himself could be summoned as a servant. His magic from the Potter world might interact in interesting ways with the rules of the Fate system. He could use spells like Expelliarmus against other servants, and his ability to resist the Dark Arts might give him an edge in battles.