They could meet during a space - time anomaly. For instance, Hermione could be pulled into the Star Wars Rebels' universe. She would use her intelligence and knowledge of magic to assist the rebels in their fight against the Empire. She might start by analyzing the Empire's technology with a magical perspective, which could give the rebels an edge.
In fanfiction, a character like Harry Potter himself could be transported to the Star Wars Rebels' setting. He might initially be confused by the new environment but quickly realize the similarities between the fight against the Dark Side in his world and the fight against the Empire. He could then use his skills in Defense Against the Dark Arts to combat the Sith - like figures in the Star Wars universe, perhaps even training some of the rebels in these combat techniques.
They could interact through a magical portal. For instance, Hermione could accidentally open a portal to the Star Wars universe while doing some advanced spell research. Then she might meet Han Solo and together they could start an adventure to find a way to close the portal before it causes chaos in both universes.
They can interact through time - travel. A device or a strange Force - related phenomenon could send a character from one era to the other. For instance, Ezra Bridger could be sent back to the Clone Wars and meet Anakin Skywalker. They could learn from each other's skills and knowledge. Ezra could learn about Anakin's aggressive lightsaber combat style, while Anakin could be intrigued by Ezra's ability to connect with different creatures through the Force.
You can start by looking on fanfiction websites like FanFiction.net or Archive of Our Own (AO3). Just search for 'Star Wars Rebels Harry Potter crossover' and you should get a list of relevant stories.
The Sith in Star Wars would be a big threat in a combined fanfic. They would likely see the magic in the wizarding world as a new source of power to corrupt. For example, Voldemort and Palpatine could form an alliance. The Jedi and the good wizards would have to put aside their differences and work together. The Jedi's combat skills and connection to the Force, combined with the wizards' magical spells, would be the only way to defeat such a powerful alliance. There would be a lot of drama and action as they fight against the combined might of the Sith and the Death Eaters.
One idea could be to have a wizard from the Harry Potter universe accidentally end up in the Star Wars Rebels' galaxy. Maybe they use their magic to help the rebels in unexpected ways, like creating magical shields or using apparition to outmaneuver the Empire's forces.
Luke would probably be in awe of Kanan's skills as a Jedi, despite his different training. He might learn some new tricks from Kanan. With Ezra, he could be like a mentor, guiding him further in the ways of the Force. And with Hera, he'd respect her leadership and might offer his own strategic insights based on his experiences fighting the Empire.
There are some really interesting crossovers in the fanfiction world. One might imagine Jedi from Star Wars Rebels visiting Hogwarts in the Harry Potter universe. It could be a great story where the Force meets magic.
The characters can be combined by creating scenarios where they meet. For example, a magical portal could open between Hogwarts and a Star Wars planet. Then, characters like Harry Potter might be attracted to the unique qualities of characters like Princess Leia or Luke Skywalker, forming part of a harem. Or a Sith Lord could find the magic of the Harry Potter world fascinating and start to court some of the powerful witches or wizards there.
Hermione Granger and Hera Syndulla are often crossed over. Hermione's intelligence and vast knowledge of magic can be paired well with Hera's leadership and piloting skills. Also, Han Solo and Ron Weasley are popular choices. Their roguish charm and sense of humor are quite comparable, and fanfiction writers like to explore how they would get along or what kind of adventures they would have together.
In a fanfiction, a character like Victor Frankenstein could be transported to Hogwarts. He would be both amazed and confused by the magic there. He might interact with Dumbledore, who could be intrigued by Frankenstein's knowledge of creating life through non - magical means. Meanwhile, the students might be either in awe or afraid of him. The Gryffindor students could try to show him the positive side of magic, while Slytherin students might be more interested in his dark secrets related to creating life.