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.
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.
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.
She could be a magical consultant. Since she has extensive knowledge of magic, she can advise the Justice League on how to deal with magical foes. For instance, when facing a villain who uses dark magic, she can tell the League about the possible weaknesses of such magic.
Perhaps in the fanfiction, Voldemort teams up with some of the Justice League's villains. This would force the heroes from both universes to work together. The wizards could teach the superheroes about magical defenses, and the superheroes could share their combat and strategic skills. Also, imagine Hermione tutoring Batman on advanced magical theories. It would be really cool to see how their different worlds' knowledge would interact.
Fanfiction often takes creative liberties. In 'harry potter justice league slash fanfiction', it could be about how the magical world of Harry Potter and the superhero world of Justice League collide. Maybe the wizards help the superheroes with some magic - related problems, or vice versa. For example, Superman could be intrigued by the magic spells in Harry Potter and try to understand them, while Harry and his friends might be in awe of the superpowers of the Justice League members.
One common theme is the clash of different worlds. The magical world of Harry Potter and the superhero - filled world of the Justice League coming together. Another theme is often the development of relationships within the harem, like how different characters interact and form bonds. And there's usually some kind of adventure or conflict that the characters have to face together.
He could be a valuable asset. His immortality means he has a vast amount of experience. He could use his magic to heal the injured members of the League quickly. For instance, with a spell like 'Episkey', he could mend wounds in seconds.