One possible storyline could be a Jedi traveling to the Wild West. He might use his lightsaber to protect a small town from outlaws, just like how he would protect a planet in the Star Wars universe. Maybe he'd also encounter some local Force - like powers that are unique to the Western setting.
A common theme is the relationship between characters. In Star Wars fanfiction, there's often a lot about the relationship between Rey and Kylo Ren, whether it's a romantic angle or a more complex friendship - enemy dynamic. In Western fanfiction, relationships like that between a rancher and a drifter can be explored, especially how they influence each other's fates. And also, the theme of redemption is popular. A Star Wars character like Darth Vader's journey towards redemption can be further explored in fanfiction, and in Westerns, a former outlaw trying to make amends.
One interesting concept could be the combination of the different technologies. For example, the warp drive from Star Trek and the hyperdrive from Star Wars. It would be fascinating to see how they interact and if they could be used together in some way. Another concept might be the meeting of the different species. The Vulcans from Star Trek and the Wookiees from Star Wars could have some very unique interactions. They have such different cultures and ways of life that it would create a lot of interesting storylines.
One common theme is the clash of motives. Mercenaries are usually in it for money or personal gain, while the Star Wars characters often have grander ideals like freedom or the balance of the Force. Another theme is the power struggle. Mercenaries may try to gain power within the existing power structures of the Star Wars universe.
In such a fanfiction, perhaps the Jedi could encounter alien species from UFOs. They might team up to fight a common enemy, like the Sith who are trying to harness the advanced technology of the UFOs for evil purposes.
The Jedi could be seen as wandering gunslingers with a moral code similar to the Jedi Code. They use their lightsabers like they would use guns, but with more finesse and a touch of the Force. For example, Luke Skywalker could be a young gunslinger learning about his powers.
For Star Wars fanfiction, start by really knowing the lore. Watch all the movies, read the books and comics. Then pick a character or a time period that interests you, like the Clone Wars era. For Western fanfiction, study the typical settings, like saloons, ranches, and deserts. Decide if you want to base your story on a real - life Western event or create a fictional town. And then just start writing, don't worry too much about grammar at first.
Some 'Star Wars meets Mercenaries' fanfictions focus on the character development of the mercenaries. For example, a story could be about a young and idealistic mercenary who starts off working for the wrong side in the Star Wars galaxy but gradually realizes the true nature of the conflict and switches allegiances. This type of fanfiction gives a fresh perspective on the Star Wars lore from an outsider's point of view.
Well, in such a fanfiction, we might see Darth Vader facing off against Voldemort. Vader's dark side powers and his imposing presence could be a match for Voldemort's dark magic. And perhaps Luke Skywalker could befriend Harry and help him in his fight against the Death Eaters. The Millenium Falcon could also fly over Hogwarts, which would be an amazing sight.