How about a crossover with the Mandalorian? The Mandalorian's no - nonsense attitude and combat skills would blend well with the Rebels. They could join forces to protect a planet or valuable artifact from the Empire. It would be interesting to see how their different cultures and motives interact in the face of a common enemy.
A crossover with the Clone Wars could be really cool. Picture some of the Clone Wars era Jedi interacting with the Rebels characters. They could share their different experiences in fighting for the galaxy, and perhaps the Rebels could learn some new combat or strategic techniques from the Clone Wars veterans.
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 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 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 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.
A fanfiction could revolve around Echo's memories. He has flashbacks to his time in the Clone Wars that end up providing valuable insights for the Rebels. For example, he remembers a weakness in a type of Imperial droid that the Rebels are currently facing. This not only helps them in battle but also makes the Rebels see Echo as a valuable asset.
Perhaps there could be a lost Mandalorian artifact that the Rebels accidentally discover. This draws the attention of a group of Mandalorians who think the Rebels are trying to steal it. As a result, they have to work together to figure out the true purpose of the artifact and face off against a common enemy who also wants it for their own evil plans.
One interesting idea could be the technology exchange. For example, the Star Trek's warp drive might be compared or even combined with the Star Wars' hyperdrive in some creative ways. Another could be the meeting of characters. Imagine Captain Kirk facing off against Darth Vader. It would be a clash of two very different leadership styles and moral compasses.
One plot idea could be Kanan and Hera getting stranded on a desolate planet and having to rely on each other to survive. They could face dangerous native creatures and have to use their unique skills to build shelter and find food. Another idea is a story where they go undercover together on an Imperial - controlled world, pretending to be a couple to gather intelligence.
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?