Ahsoka Tano would be an excellent addition. She has a rich history in the Star Wars universe, from the Clone Wars to her later appearances. Her wisdom and combat skills could be a great asset to the Rebels in a crossover. She could also provide a link between different eras of the Star Wars story.
Boba Fett would be a cool character to throw into a Star Wars Rebels crossover fanfic. His bounty hunter skills and his own vendettas could create some interesting plotlines. Maybe he's hired by the Empire to take out the Rebels, but then has a change of heart and ends up helping them instead.
Naruto and Ezra would likely be main characters. They are both young heroes on a journey of self - discovery.
Definitely include Obi - Wan Kenobi. He has a connection to Geonosis already from the movies. Anakin Skywalker would also be a great addition. And for a non - Jedi character, you could add a Geonosian rebel who is trying to overthrow the corrupt leaders on the planet.
Well, Sabine Wren from Star Wars Rebels would be a great candidate. She has a strong connection to Mandalorian culture already. She could interact with Din Djarin from The Mandalorian. They could exchange stories about their experiences with Mandalorian traditions and perhaps even team up to fight against the Empire.
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.
Han Solo and a Skaven from Warhammer. Han's roguish charm and ability to get out of tough situations could be pitted against the devious and sneaky Skaven. The Skaven's underhanded tactics and Han's quick thinking would make for some really entertaining encounters as Han tries to outwit the Skaven in various situations across the two universes.
You could include characters from other crime - themed games like Tommy Vercetti from Grand Theft Auto: Vice City. His experience in the crime world could create some interesting dynamics with the Mafia 3 characters. They could either be rivals or form an uneasy alliance.
I think pairing Kanan Jarrus and Dumbledore would be really interesting. Dumbledore is this wise and powerful wizard, and Kanan is a Jedi with his own set of skills and knowledge. They could have deep discussions about leadership, the nature of good and evil, and how to train the younger generation. Kanan could learn from Dumbledore's teaching methods when it comes to mentoring Ezra, and Dumbledore could gain insights into the Force from Kanan.
Definitely Indiana Jones and Han Solo. Indiana's tenacity and knowledge of archaeology combined with Han's roguish charm and piloting skills would make for some great interactions. They could be on a quest for a valuable artifact that has significance in both the Star Wars galaxy and on Earth.
From other franchises, Link from The Legend of Zelda could be an interesting addition. His sword - fighting skills and puzzle - solving abilities would contrast with Sonic's speed - based combat. He could be on a quest in Sonic's world and end up teaming up with Sonic to defeat a common enemy. Another character could be Samus Aran from Metroid. Her powerful weapons and armored suit could bring a new level of badassery to the fanfic, and she could have some interesting interactions with Sonic and his friends.
In a crossover with Sailor Moon, Sailor Moon herself would be a great addition. She has her own powers and a strong sense of justice, just like Ladybug. Tuxedo Mask could also be interesting to pair with Cat Noir. Another good character could be Naruto from the Naruto universe. His ninja skills and determination would blend well with the Miraculous Ladybug world.