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.
There are often fanfics that explore the cultural differences between Jack and the Scotsman's daughters. For example, Jack could introduce them to new ideas or ways of life, while they in turn could teach him about their Scottish heritage. This can lead to really interesting storylines filled with character growth and discovery.
You can try some popular fanfic websites like Archive of Our Own (AO3). It has a large collection of fanfics on various topics, so there's a good chance you'll find some related to Jack and the Scotsman's daughters there.
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.
It depends. Some consider them canon if they fit seamlessly into the established Samurai Jack universe and don't contradict the main storylines. Others might have different opinions.