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.
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.
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.
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.
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.
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.
He could be a strategic addition. His magic could be used for things like stealth, teleportation, and protection spells for the team. For instance, he could teleport members in and out of dangerous situations quickly.
Well, one way could be that Fem Harry Potter, with her magical skills, could bring a unique perspective to the Justice League. Maybe she uses her spells to enhance the League's defenses or to help them in battles against magical threats. For example, she could create protective shields around the League's headquarters using a combination of her magic and the technology available in the Justice League universe.
If we consider the harem aspect first, perhaps there's a powerful character who has the ability to draw characters from both the Justice League and Harry Potter series into their 'harem'. The 'lemon' could be part of a magical or scientific experiment that links these two disparate universes. For instance, it could be a lemon - shaped device that opens portals between the two worlds. And in this new combined world, the characters have to navigate their relationships and powers while dealing with the challenges that come with this strange new setup.
The plot might involve the Justice League finding a way to incorporate magic into their understanding of the world because of Harry. For example, the Flash could be curious about how Harry can apparate and might try to find a scientific explanation for it. Aquaman could teach Harry about different forms of communication, as he has his own unique way of communicating underwater. As Harry grows, he might use the Justice League's resources to fight against the Death Eaters. He could bring some of the League's technology into the wizarding world, which would change the dynamics of the fight against Voldemort.
His character would develop a more global perspective. Instead of just focusing on the wizarding world's problems, he'd be aware of the wider universe of threats that the Justice League faces. This could make him more open - minded and innovative in his approach to solving problems. He might also be more physically fit and combat - ready, having trained with Batman and others. His relationships with others would be different too. He might not have the same close - knit group of friends as in Hogwarts, but rather a more diverse group of allies from different backgrounds, like the various members of the Justice League.