Yes, there are quite a few. You can start by looking on fanfiction websites like Archive of Our Own or FanFiction.net. Just search for the key terms 'Harry Potter Justice League Harem' and you'll likely find some interesting reads. However, the quality can vary greatly, so it might take some time to find one that really suits your taste.
In a Naruto in Justice League fanfiction scenario, Naruto would have a lot to offer. He has a vast amount of chakra which could be used for various purposes like powering up the League's technology or creating barriers. His relationship with his teammates in Naruto like Sasuke and Sakura could be paralleled with the relationships in the Justice League. Naruto might have some initial cultural clashes with the League members as he comes from a different world, but over time, he would learn to adapt and they would learn from his unique skills and way of thinking. His Sage Mode would give him an edge in battles, allowing him to sense danger more acutely and react faster.
One great 'Young Justice Dimension: Justice League' fanfiction is 'The New Recruits'. It focuses on how new members are integrated into the team. The story has great character development and exciting action scenes.
The Flash might initially be in denial. He's used to Batman always having a plan and his death would be a huge shock. He would then turn his speed into a tool for investigation, trying to find out exactly what happened. Green Lantern could feel a sense of loss but also be spurred into action, using his ring to try and prevent future tragedies like Batman's death. He might create constructs in Batman's memory to inspire the team.
One way it could work is by having the magical elements of Harry Potter in the Justice League universe. For example, the wizards could help the Justice League with their spells in battles against powerful foes. Maybe Harry Potter himself could use his Patronus charm to fend off some of the League's enemies like Darkseid's minions.
It could be a wild adventure where Harry Potter, with his magical abilities, teams up with the Justice League. Maybe Harry uses his spells to enhance the League's powers or to combat enemies that are immune to their usual tactics. For example, he could use a Patronus charm to fend off some dark, mind - controlling villains that the League has trouble with.
Well, if Fem Harry was in a Justice League fanfiction, the storyline could be very different. For one, her character might bring a different dynamic to the team. She could have a different approach to problem - solving compared to the male Harry. Maybe she'd be more in tune with the emotional side of things within the team, which could lead to different relationships among the Justice League members.
Well, in a fanfiction where Harry Potter joins the Justice League, there would be some interesting interactions. Harry's knowledge of the magical world could help the Justice League deal with threats that have a magical or otherworldly origin. His experience in fighting dark wizards might give the team new strategies. Also, the members of the Justice League would probably be fascinated by his magic, and there could be some funny moments of them trying to understand how it all works.
A really cool crossover concept could be a combined threat that requires both the skills of the Justice League and the magic of the Harry Potter world to defeat. Maybe a powerful alien sorcerer arrives on Earth. The Justice League's superpowers alone aren't enough, so they team up with Harry Potter and his friends. Harry could use the Patronus charm to fend off the dark magic of the alien, while the Flash could use his speed to quickly deliver magic items to different locations as needed.
Imagine a scenario where a young wizard from the Harry Potter world, who has a natural ability to open portals, accidentally opens a gateway to the world of the Justice League. However, this also attracts the attention of Silent Hill. The League and the wizards have to work together to close the portals and keep Silent Hill's nightmares at bay. The wizards can use their spells and the League can use their superpowers to fend off the encroaching darkness.