A new original character could be created. For example, a young journalist who gets involved with the Justice League's adventures. She's smart, brave, and has a heart of gold. Her presence starts to make the male heroes feel things they haven't felt before, and she becomes the focus of their affections.
Lois Lane could also be a possibility. Although she's mainly associated with Superman, in a harem - type scenario, she could interact more with other members of the Justice League. Her feisty personality and her importance in the DC universe could make her an interesting main female character in this kind of fanfiction.
Wonder Woman could be a great choice. Since she's already part of the Justice League, she could be the central female figure in a harem - style story. Her strength, wisdom, and beauty would attract the other male members of the League, and her own complex personality would add depth to the relationships.
Definitely Wonder Woman. She is a powerful and iconic female superhero. Her strength and sense of justice would make her an interesting character to interact with Naruto. Also, Hawkgirl with her fierce fighting skills and strong will. And then there's Supergirl. She has a lot in common with Naruto in terms of having great power and the responsibility that comes with it. These three could be the main female characters in such a fanfiction.
It could be a combined force of Voldemort and one of the major Justice League villains like Darkseid. Voldemort might be attracted to Darkseid's power and they form an alliance. Their goal could be to take over both the wizarding world and the world where the Justice League exists. They would use their combined magic and super - villain powers to create chaos and destruction.
A tech - genius named Techno - Tyrant. He could be a former game developer who turned evil after his creations were rejected. He uses his knowledge of gaming mechanics and virtual reality to create dangerous traps for the Justice League.
Darkseid as a vampire - like being. He could come to Earth and start turning people into his vampire minions. His already formidable powers combined with vampiric abilities would make him a terrifying foe for the Justice League.
Lex Luthor could be a great main antagonist. He might see Izuku as a threat to his plans, especially if Izuku's unique abilities and sense of justice start to disrupt Luthor's schemes in the combined universe. Luthor could try to manipulate events to turn the Justice League against Izuku or find a way to steal Izuku's power for himself.
Obviously Batman himself is a key character. Then there could be characters like Catwoman, who has a long - standing relationship with Batman. Wonder Woman could also be part of the harem, given her strong connection to Batman in some storylines. And perhaps Batgirl, as she has a close relationship with Batman in the comics.
Well, perhaps in this fanfiction, there's a common enemy that threatens both Naruto's world and the Justice League's world. Naruto teams up with the female heroes. During their adventures together, they learn about each other's cultures and values. The female heroes are intrigued by Naruto's ability to use chakra and his ninja way. This mutual admiration could lead to a harem - like relationship where they all care for Naruto in different ways.
It could be really interesting. Spiderman in the Justice League setting would bring a different dynamic. In a harem scenario, there could be various relationships with other Justice League members. Maybe he'd have a close bond with Wonder Woman, sharing his street - level hero insights with her more high - concept heroism. And with Flash, they could have some fun, friendly rivalries while also being part of this harem - like group.
It could work if there was a common enemy that forced them together. In the fanfiction, this enemy might be so powerful that they need to combine forces. And as for the harem aspect, it could be that Spider - Man's charm and his outsider - status make him appealing to some of the Justice League members. They might see him as something new and exciting, different from their usual comrades. So, they gradually develop feelings for him.
It could be a new villain who has a grudge against the Justice League. Maybe someone whose family was affected by the League's actions in the past. This villain would be very smart and powerful, creating elaborate traps for the League members.