A story where the Tau from Warhammer 40k make contact with the more diplomatic species in Star Wars, like the Mon Calamari. The Tau's ideology of the Greater Good could either clash or find some common ground with the ideals of the Star Wars species. It would be fascinating to see how they interact and whether they can form an alliance or if it all falls apart due to cultural differences.
Perhaps an Imperial Inquisitor ends up in the Star Wars galaxy and tries to manipulate the Sith and Jedi for his own ends. He could see the Sith as a useful tool, not realizing how dangerous they truly are until it's almost too late. This could lead to a three - way battle between the Inquisitor, the Sith, and the Jedi.
Another interesting concept is a crossover during a time of great chaos. For example, the warp rifts in Warhammer open up near a key Star Wars system. This causes all kinds of chaos as the daemons from Warhammer pour into the Star Wars universe, and the various factions in Star Wars have to figure out how to deal with these otherworldly threats. It could also explore how the Jedi might try to use the Force to combat the daemonic influence.
One interesting plot twist could be the combination of the Force from Star Wars with the psychic powers in Warhammer 40k. Imagine a Jedi using his Force powers in the grim and dark universe of 40k, facing off against Chaos psykers. Another could be the appearance of Imperial Stormtroopers in a Warhammer 40k battle, their white armor standing out among the chaos. Maybe they could be mistaken for some sort of new xenos threat at first. And a third plot twist might be the discovery of a lost Star Wars planet in the Warhammer 40k galaxy, full of unique technology and species that both sides would be eager to control or destroy.
A great plot could be that Master Chief somehow gets transported to the Star Wars universe. He could find himself in the middle of a battle between the Sith and the Jedi. His combat skills and advanced armor would be a huge asset to either side. Or perhaps there could be a story where the Forerunners have some ancient connection to the Force in the Star Wars galaxy. This could lead to a search for lost Forerunner technology that could change the balance of power in the Star Wars universe.
Perhaps in a Star Wars Twilight crossover fanfiction, the Force could be seen as a kind of super - power in the Twilight universe. A Jedi could come to train the Twilight characters in how to use this 'new' power. They would have to adapt their Jedi teachings to fit the different nature of the Twilight world, which has its own set of rules regarding vampires and werewolves.
One plot idea could be Superman being transported to the Star Wars universe. He might use his powers to help the Rebel Alliance. For example, his super strength could be used to move large objects during battles or his heat vision to take out enemy ships. Another idea is that he teams up with Jedi. He could learn about the Force and they could combine their powers to fight the Sith.
One idea could be Jedi coming to Earth to train a select group of humans with Force - like abilities. Another could be a Sith Lord arriving on Earth and causing chaos, and Earth's heroes have to team up with Star Wars characters to stop him. Maybe there's a discovery of a hidden Star Wars - related artifact on Earth that has the power to change the balance of the Force.
One plot idea could be Thrawn crossing over into an alternate Star Wars timeline where the Jedi Order never fell. He could find himself in a situation where he has to adapt to a galaxy full of powerful Jedi. Another idea is a crossover with a different sci - fi universe, like Star Trek. Thrawn might use his tactical genius to face off against the Federation. Or perhaps a crossover with a post - apocalyptic Star Wars setting, where Thrawn has to lead a group of survivors against new threats.
One idea could be that the Spartans from Halo are sent through a wormhole and end up in the Star Wars galaxy. They could then get involved in the Clone Wars, with their advanced technology and combat skills being a huge asset or a source of great confusion among the different factions. Maybe they team up with the Jedi at first, but then some misunderstandings occur due to their different ways of handling situations.
One idea could be a group of Guardians from Destiny being transported to the Star Wars universe. They could team up with the Jedi to fight against the Sith and the Darkness that threatens both universes. Maybe they would introduce their advanced weaponry and abilities to the Star Wars characters.
Well, a really cool plot could be that the Frozen world is actually a long - lost planet in the Star Wars galaxy. The characters from Frozen get involved in the Galactic Civil War. Maybe Kristoff, who is used to cold environments, helps the Rebel Alliance on an ice - covered planet. Elsa could also use her ice powers to create defenses against the Empire's attacks, like building ice walls around Rebel bases. And Anna could befriend some of the alien species in the Star Wars universe and convince them to join the fight against the Empire.