Well, it could be as simple as using the themes of heroism from '911'. In the Star Wars universe, you can have characters show the same kind of selfless heroism as the firefighters and police officers during '911'. For example, a group of rebels might sacrifice themselves to save a planet in a similar way.
Another aspect to consider is the story arcs. Just as in Star Wars Rebels there are character developments and plot twists, in a WW2 - inspired fanfiction, these could be based on real historical events. For instance, a character could be inspired by the story of Dunkirk, where they face seemingly impossible odds but manage to survive and come back stronger, much like the Rebels in their battles against the Empire.
One way to combine them could be to set a Star Wars story in a universe with the aesthetic of Heavy Metal 1981. For example, imagine a Star Wars story where the technology has a more gritty, industrial look like in Heavy Metal. The characters could have a darker, more edgy vibe. Maybe the Sith lords in this fanfic have a more heavy metal - inspired appearance, with spiky armor and wild hair.
In terms of storylines, a fanfiction could focus on a mission where Naruto and his friends are transported to the Star Wars galaxy during the Clone Wars. They might team up with Jedi and clones to fight against the Separatists. Naruto could learn about the Jedi way and share his ninja philosophy. This would create an interesting dynamic as the two different cultures of ninjas and Jedi interact.
One way to combine them is to have Naruto characters in a Star Wars - like universe. For example, Naruto could be a Jedi - like figure with his chakra being like the Force. The 'King' could be a powerful Sith - like character who rules over a part of the galaxy. Fanfiction writers can play with the concepts of Naruto's ninja skills and Star Wars' space - faring technology to create unique and exciting stories.
You can have characters like Ash from Pokemon become a part of the Star Wars universe. He could be a young Force - sensitive being who uses his knowledge of Pokemon to his advantage. For instance, his Pokemon could help him in starship battles or in exploring new planets.
One way is to have the Star Wars characters time - travel to WW2. For example, Darth Vader could end up in a battle and his advanced technology and powers would shock the soldiers. It would be a cool scenario where the dark side of the Force meets the real - world chaos of war.
The characters can be combined by having them team up. For example, Naruto and Luke Skywalker could join forces. Naruto's energy and determination along with Luke's connection to the Force would make a powerful duo. They could face common enemies like the Sith or rogue ninja. Another way is to have them teach each other. A Jedi could teach a ninja about the Force, and a ninja could teach a Jedi some ninja techniques.
One way to combine them is by having a crossover where a US Army soldier somehow gets transported to the Star Wars universe. Maybe through a strange portal or experimental technology. He could then use his military training in the new and dangerous environment of Star Wars, like in combat against the Sith or in strategic planning with the Rebel Alliance.
One way to combine them creatively could be to have a Naruto - like character in the Star Wars universe who becomes a senator. For example, a ninja - skilled character from Naruto's world finds themselves in the Galactic Senate. They could use their unique abilities like stealth and combat skills to deal with political intrigue and threats within the Senate, much like how senators in Star Wars face various challenges.
One way to combine them creatively could be to have a character with Sharingan-like abilities in the Star Wars universe. For example, a Jedi or Sith might develop an ability similar to the Sharingan, which allows them to see the flow of the Force in a unique way, predict their opponents' moves better, and even copy some Force techniques. In a Naruto - Star Wars crossover fanfiction, Naruto himself could be transported to the Star Wars galaxy and his chakra-based powers could be reimagined as a new form of Force ability, with his Sharingan - like dojutsu being a rare and powerful gift that confounds the Jedi and Sith alike.