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 way is through flashbacks. Maybe Kanan or Hera have heard stories from older Jedi about Yoda. These stories could then be shown in flashbacks and influence their actions in the present. Also, Yoda's teachings could be passed down through Jedi texts that the Rebels find. This would be a more indirect but still realistic way of incorporating him into the story. And his connection to the Force could be felt by Force - sensitive Rebels in times of great need, guiding them without actually appearing physically.
Another interesting plot could involve Yoda being seduced by the dark side much earlier in his life. He then tries to recruit other Jedi to his new Sith ways, leading to internal conflicts within the Jedi Order. This could show a different side of the power dynamics and relationships in the Star Wars galaxy.
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 plot idea could be that a character from Clone Wars gets accidentally sent to the time period of Rebels. For example, Ahsoka Tano could end up in the middle of a battle between the Rebels and the Empire. She might use her knowledge from the Clone Wars era to help the Rebels in unexpected ways, like teaching them some old Jedi combat techniques that were lost over time.
One interesting concept is exploring the backstories of minor characters. For example, in some fanfictions, they might delve into the past of a random stormtrooper, showing how they ended up in the Empire. Another concept could be alternate endings to major events in the series. It gives fans a chance to rewrite parts they thought could have been better.
One great Star Wars Rebels Rex fanfiction is 'Rex's Redemption'. It shows Rex finding his place in the Rebellion after his Clone Wars experiences. Another is 'Rex and Ezra's Bond', which focuses on the relationship between Rex and Ezra Bridger. There's also 'Rex's Leadership in the Rebels' that details how Rex uses his military knowledge to help the Rebels.
A plot could be that the OC is a long - lost relative of Ezra's. They meet during a battle and initially don't realize their connection. As they get to know each other, they start to notice similarities in their abilities and appearance. Eventually, they discover their family tie and it strengthens their bond within the Rebellion.
One plot idea could be exploring the backstory of a minor character from the 'Spark of Rebellion' episode. Maybe they had a family on a planet that was being oppressed by the Empire and their journey to join the Rebellion was filled with personal challenges and sacrifices.