It could be Aku. Since he is the main villain in Samurai Jack's world, his power and evil nature would make him a great antagonist. He could see Harry's magic as a threat and try to eliminate him. And if he teams up with some of the dark forces from the Harry Potter universe like Voldemort or the Death Eaters, it would be even more challenging for Harry and Jack.
One possible plot could be that Harry Potter is somehow transported to the world of Samurai Jack. He teams up with Jack to fight Aku. Harry uses his magic, like the Patronus charm to fend off Aku's dark minions, while Jack uses his sword skills. Their combined powers and different fighting styles make them a formidable duo against Aku's tyranny.
Maybe there could be a crossover where some of the characters from Harry Potter, like Hermione, are fascinated by the technology and magic in Samurai Jack's world. They could study the different forms of magic there and try to incorporate it into their own magical knowledge. And in return, they could teach Jack some basic spells to use against Aku.
First, do a lot of research on both universes. Know the rules of magic in Harry Potter and the lore of Samurai Jack. This will help you create a believable crossover. For example, understand how spells work in Harry Potter and what Jack's sword can and can't do.
Well, it could be that Jack Frost accidentally ends up in the wizarding world. Maybe he freezes some of the Hogwarts grounds and Harry and his friends have to figure out how to unfreeze it without magic harming Jack.
Well, perhaps in this fanfiction, Laughing Jack somehow finds his way into Hogwarts. He starts causing chaos in the school, like making the staircases move erratically or turning the portraits into creepy, distorted versions of themselves. Harry Potter then has to figure out a way to stop Laughing Jack without using dark magic. He might seek help from Dumbledore and use his knowledge of both the wizarding world and his own courage to face this new threat.
Yes, there are quite a few out there. Some might be found on popular fanfiction websites like Fanfiction.net or Archive of Our Own. You can search for specific tags like 'Jack Frost crossover Harry Potter' to narrow down the results.
One great Samurai Jack fanfiction story is 'The Lost Years of Jack'. It delves into what Jack might have been through during his time away from his home. It's really interesting as it shows his struggles and how he managed to keep his spirit alive in the face of so many challenges.
I think Jack Frost would be the most interesting. He comes from a completely different world of ice and cold, and his powers are so different from the magic in the wizarding world. His carefree and mischievous nature would contrast well with the more rule - bound wizarding students at Hogwarts.
Yes, there might be. The 'Harry Potter' and 'Eyeless Jack' fandoms are both quite large, and it's not uncommon for fans to create crossover fanfictions. However, the nature of such a crossover can be very strange as they come from different fictional universes with very different themes.