A natural choice for a main character could be Spartacus himself. Transplanted into the Star Wars universe, he could be a freedom fighter against the Empire. He could inspire the downtrodden species in the galaxy with his unwavering will for freedom. Another could be Anakin Skywalker. If he were to meet Spartacus, he might learn different perspectives on leadership and freedom. Spartacus could show Anakin that true power comes from the people, not from the Sith Lords.
One possible plot could be that a group of gladiators from the Spartacus era are somehow transported to the Star Wars universe. Maybe they end up on a planet like Tatooine. The gladiators' combat skills would be highly valued in the Star Wars underworld. They could get involved in the local power struggles, perhaps siding with the Rebels against the Empire as they are used to fighting for freedom in their own time.
Starkiller can be a game - changer in Star Wars crossover fanfiction. He can bring his combat skills and Force knowledge to new settings. For example, if he's in a crossover with the Matrix universe, his ability to sense and manipulate the Force could be compared to Neo's ability to control the Matrix.
One possible fanfiction could be about a Sith Lord from Star Wars ending up in the Naruto universe. Maybe he tries to use the Force to control the chakra, but finds that it's a completely different power system. He could then form an alliance with some of the villains in Naruto, like Orochimaru, who are also interested in power and dark arts.
The cultures in both universes are also ripe for exploration in the crossover. The Federation's values of equality and cooperation in Star Trek contrast with the more hierarchical and often morally ambiguous cultures in Star Wars. A story could focus on how characters from each side learn from or clash with the other's cultural norms.
The plot might involve Spartacus and his men seeking the help of the Spartans from '300'. The Spartans are initially hesitant as they have their own ways and beliefs. But as they see the oppression that the Romans are inflicting on Spartacus' group, they decide to help. Along the way, there are clashes in cultures and values, but also a growing respect between the two groups. This could lead to a final epic battle against the Romans where they use both Spartan and gladiator fighting strategies.
Well, on Archive of Our Own (AO3), there are a few. In these crossovers, often the different fighting styles of the '300' warriors and the 'Spartacus' characters are compared and contrasted. It can be really cool to see how the honor - bound Spartans deal with the more rebellious nature of the Spartacus crew.
The fanfiction 'Rey Meets the Avengers' is also a good one. In this story, Rey teams up with the Avengers. Her lightsaber skills are a huge asset when facing off against the villains. She has some really cool interactions with Iron Man, as they both are sort of tech - savvy in their own ways.
A great idea for an Echo x Star Wars Rebels crossover fanfiction would be that Echo has some sort of encrypted data in his cybernetic enhancements that could be crucial for the Rebels. The Rebels have to figure out a way to access it without harming Echo. This could lead to a lot of character - development moments as they get to know Echo and his past better, and Echo gets to adapt to the Rebels' way of doing things.
One great Star Wars Transformers crossover fanfiction is 'The Cybernetic Jedi'. In this story, Optimus Prime teams up with Luke Skywalker. It combines the heroism of the Jedi with the power of the Autobots in a really cool way. The battles are epic as they face off against the Decepticons and the Sith together.