One possible main character could be an Earth scientist. They would be in a unique position to study the Empire's technology and try to find a way to defend Earth.
An Earth politician could be a main character. They would have to deal with the diplomatic nightmare of an alien empire arriving on their planet. They would need to negotiate with the Empire, try to understand their motives, and at the same time figure out how to protect Earth's interests and sovereignty.
In this kind of fanfiction, the Empire's arrival on Earth would change everything. They might be interested in Earth's biological diversity for their own experiments. The Jedi, if they somehow got involved, could either help Earth or have their own agenda. Also, the political situation on Earth would be turned upside down. Nations might have to unite against the common threat of the Empire. And there could be interesting character interactions, like an Earthling befriending a stormtrooper who starts to question the Empire's ways.
A character could be a Force - sensitive child from the United Earth Empire. He was born with this power but was hidden away on Earth. When the Empire discovers his abilities, they want to use him as a weapon. However, he is befriended by a group of rebels who help him understand the true nature of the Force and fight against the Empire's evil plans.
It could be an ordinary human on Earth who rises to become a hero against the Empire. Maybe a scientist who figures out a way to counter the Empire's force powers.
One main character could be a droid technician working for the Empire. They might be involved in creating new and more powerful droids for the Imperial military. A Twi'lek slave who has been forced to serve in an Imperial household could also be a main character. This character could be plotting their escape while also learning secrets about the Empire's inner workings. Additionally, a Mandalorian who has made a deal with the Empire to gain certain resources or advantages. But as time passes, they realize the true cost of serving the Empire.
An experimental subject, a Force - sensitive being who manages to break free from the Empire's control and then becomes a key figure in fighting against the Empire's evil plans.
The droids in Star Wars could be used by the Earth Empire for various tasks. However, they might also face issues like compatibility with Earth's power sources. And the hyperdrive technology from Star Wars could revolutionize Earth Empire's space exploration capabilities if they could figure out how to integrate it into their existing spacecraft.
A young Earthling who is discovered to be extremely Force - sensitive could be a main character. He or she could be mentored by an old Jedi master and go on a journey to learn about the Force and their place in the Star Wars universe.
One main character could be a military general on Earth. He has extensive combat experience and is tasked with leading the defense against the Star Wars invaders. Another could be a young orphan who discovers he has an uncanny ability to interact with the Force, which makes him a key figure in the fight. And perhaps a Jedi who has been exiled to Earth long ago and now has to step up to help his new home planet.
Maybe an experienced admiral from the Earth Defense Fleet. She would bring her strategic knowledge to the table when dealing with the complex political and military situations in the Star Wars universe. And she could also form alliances with characters like Leia Organa.
A likely main character could be a scientist from Earth who is excited about the opportunity to study the advanced technology of the Star Wars universe. They would be in the middle of the relationship between Earth and the CIS, trying to use the alliance for scientific progress. Also, a CIS spy on Earth could be a main character, tasked with ensuring Earth's loyalty and reporting any signs of betrayal.