You could write a fan fiction where the Rebels discover a long - lost Jedi artifact that gives them an edge in the fight against the Empire. This could involve a quest to find the artifact, with various challenges and encounters along the way.
Another interesting angle is to write about the relationships between the different members of the Rebellion. Maybe a story about how two characters who initially didn't get along eventually become friends and rely on each other in battle.
There are several great plot ideas. For instance, imagine a situation where Yoda, before his death, set in motion a series of events that are only starting to unfold during the Rebels' time. Maybe he planted the seeds for a new Jedi order in a very secretive way. A young Rebel might stumble upon these secrets. Another idea is that Yoda's teachings, which were thought to be lost, are rediscovered by the Rebels through an ancient Jedi artifact. This discovery changes the way they approach the fight against the Empire. And what about a story where Yoda's influence reaches out to a non - Force - sensitive character in Rebels, inspiring them to do great things for the Rebellion in a different way?
One idea could be exploring the backstory of a minor character in the series. For example, you could write about how a Stormtrooper decided to defect and join the Rebels. Another plot could involve a new threat that the Rebels have to face, something that even the Empire is not fully aware of. Maybe a powerful ancient Sith artifact is discovered and both the Rebels and the Empire are racing to get it.
One idea could be a crossover with the original Star Wars trilogy characters. Maybe the Rebels crew teams up with Luke Skywalker and Han Solo to take on a new, powerful Sith threat. This would bring together the scrappy underdog feel of the Rebels with the classic heroism of the original trilogy.
One idea could be a story where a new, young Jedi discovers a hidden Sith artifact that has the power to control the Force in ways not seen before. This could lead to a race between the Resistance and the First Order to either obtain or destroy it.
An interesting plot could be an OC who is a mechanic with a mysterious past. They join the Rebels and start to tinker with their ships and weapons, but they also have a connection to an ancient Jedi artifact. As they work with the Rebels, they gradually uncover the secrets of the artifact and how it can be used against the Empire. Or perhaps an OC who is a youngling that was hidden away during Order 66 and now, years later, emerges to join the Rebels and seek revenge on the Empire while also learning the ways of the Force from the few remaining Jedi.
One idea could be that Master Chief somehow gets transported to the Star Wars Rebels universe. He could team up with the Ghost crew. His advanced combat skills and Spartan armor would be a huge asset against the Empire. Maybe he'd share his knowledge of advanced weaponry with the rebels.
One idea could be a crossover with the original Star Wars trilogy characters. For example, having Luke Skywalker meet the Rebels crew and learn from their guerilla warfare tactics against the Empire. It would be exciting to see how their different fighting styles blend and how they interact with each other.
One idea could be exploring the post - Rebels era and how the actions of the main characters have influenced the new generation. Maybe a young Jedi discovers an old artifact left by Ezra Bridger and it sets them on a journey to learn more about the Rebels' fight.
A character could experience a moral transformation. A former Imperial who has joined the Rebels might start to transform back to his old ways due to the stress and hardships of the Rebellion. This internal struggle could be a central part of the fanfiction. The other Rebels would then have to decide what to do with him - try to reform him again or cut their losses. It would show the complex nature of the Rebels' cause and the different types of people it attracts.
One idea could be exploring the backstories of the clone troopers in more depth. For example, showing how they interacted during their training and formed unique bonds. Another could be a story about a Jedi Padawan who gets separated from their master during a battle and has to find their way back while facing unexpected challenges. A third idea is to create a story where a group of clones discovers a hidden Sith artifact and has to decide what to do with it.