One idea could be that Voldemort discovers a way to enter the DC universe and teams up with some of the Justice League's villains like Lex Luthor. The Hogwarts students and the remaining members of the Justice League have to join forces to stop them. The wizards can use their magic while the superheroes use their powers.
Another concept is that the magic from the Harry Potter series starts to seep into the DC universe where the Justice League operates. This causes chaos as the laws of physics in the DC world are disrupted. The Justice League has to seek help from Hogwarts professors to understand and contain this new form of magic energy. Superman might be intrigued by the idea of magic being a new power source or a new threat he can't easily overcome with his super strength.
Hermione Granger and Batman would be great to focus on. Hermione is extremely intelligent and well - read in the magical lore. Batman is a brilliant detective with a vast knowledge of various sciences and combat skills. Their interaction could be really engaging. Hermione could introduce Batman to the world of magic, and Batman could show Hermione how to use her intelligence in a more practical, non - magical way in different situations. Also, Wonder Woman and Ginny Weasley could be an interesting pair. Wonder Woman with her Amazonian strength and Ginny with her powerful magic could team up and have some great adventures together, sharing their different perspectives on heroism.
First, you need to do your research on both universes. Know the characters, their powers, and their personalities well. For the harem part, decide which characters will be part of it and how their relationships will develop. Maybe start with a plot device like a magical object that links the two worlds. Then, gradually introduce the characters to each other. Write engaging dialogues that show the different cultures of the two universes.
In a 'Harry Potter Justice League crossover fanfiction', there would be a lot of interesting interactions. The wizards from Hogwarts and the superheroes of the Justice League would have to figure out how to work together. For example, Batman would be intrigued by the magic of Hogwarts and might try to analyze it scientifically. At the same time, Hermione could learn a thing or two from Batman's detective skills. And when it comes to facing a common enemy, like an alien sorcerer, they would combine their powers. The Flash could use his speed to quickly deliver magical items that the wizards need in battle, while the wizards use spells to protect the Flash from magical attacks.
Well, I think Batman and Hermione could be main characters. Batman's intelligence and detective skills combined with Hermione's vast knowledge of magic would make for an interesting dynamic. They could be the ones leading the team in strategizing against the common enemies that the crossover would bring. Also, Wonder Woman and Ginny Weasley could be important. Wonder Woman's strength and combat skills paired with Ginny's talent in magic, especially her Bat - Bogey Hex, could make them a formidable duo in the story.
One idea could be that the Justice League somehow ends up in the wizarding world. Maybe a magical portal opens during a battle in their universe and sucks them into Hogwarts. Superman's powers could be affected by the magic there, and Batman would be intrigued by the complex magic system and start studying it. Harry Potter and his friends could team up with the Justice League to fight a new dark force that threatens both the magical and non - magical worlds.
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.
A Harry Potter and Justice League harem fanfic could start with a portal opening between the two worlds. Characters like Harry Potter himself might find himself in the world of the Justice League, or vice versa. The harem part could play out in different ways. Maybe Superman, being the noble hero he is, finds himself in a situation where he has to protect multiple female characters from both universes. And as they spend more time together, feelings start to develop in a complex web of relationships. It would be a unique take on the crossover genre, exploring both the action - packed aspects of the Justice League and the magical elements of Harry Potter while adding the interesting dynamic of a harem.
Teamwork is another big theme. In these crossovers, the Justice League has to learn to cooperate with new characters. They often start off with some misunderstandings, but then they come together to face a greater threat. It's all about how different heroes can combine their skills for a common good. In many fanfics, there's also a theme of exploring new worlds. When the Justice League crosses over, they get to see different universes, cultures, and ways of doing things in the other team's world.