In the official Star Wars lore, there is no explicit romantic relationship between Ahsoka and Rex. Their relationship is mainly one of comrades and friends. They have a deep bond based on their shared experiences in the Clone Wars, fighting side by side and respecting each other's skills and loyalty.
One possible fanfiction could be about Rex and Ahsoka going on a secret mission together. They might face a new and dangerous enemy that the Republic has not yet encountered. Rex would use his military tactics and Ahsoka her Jedi skills to overcome the challenges.
A third storyline could be set during a time of great chaos in the galaxy. Ahsoka and Rex find themselves in the middle of a power struggle. They have to use their skills and knowledge to try and bring some sort of peace. Ahsoka's Jedi training and Rex's military experience would be put to the test as they navigate through political intrigue and battles against different factions.
For Ahsoka, her character could be developed further by exploring her feelings of being an ex - Jedi. She could struggle with her place in the galaxy now that she's no longer part of the Jedi Order. Rex, on the other hand, could be developed by showing his life after the Clone Wars. Maybe he has a hard time adjusting to a new way of life without the structure of the military.
Yes, there are quite a few. You can check out platforms like Archive of Our Own. Some fanfictions focus on their adventures together and how their relationship might develop in a romantic way, with beautiful descriptions of their interactions.
One common theme is loyalty. Rex and Ahsoka have a deep loyalty to each other that dates back to the Clone Wars. In many fanfictions, this loyalty is tested but always prevails. For example, in stories where they are faced with difficult choices that could put the other in danger.
One great storyline could be Ahsoka and Rex having to work together to rescue a group of captured clones. They face many challenges and enemies along the way, relying on their unique skills and friendship. Another could be them exploring an uncharted planet where they discover an ancient Sith artifact and must decide what to do with it.
Perhaps a fanfiction where Rex and Ahsoka have to infiltrate an enemy base. Ahsoka could use her Jedi mind - tricks to get past guards, while Rex would be in charge of the technical aspects like disabling security systems. It would be full of suspense as they try not to get caught.
The appeal lies in their pre - existing connection. They have a deep bond from their time in the Clone Wars. It's exciting to see that relationship turn romantic in fanfiction.
Explore their relationship in terms of loyalty. Rex was always loyal to Ahsoka, and she to him. You could write a story where that loyalty is tested. Maybe they are faced with a situation where they have to choose between each other and the greater good, and how they navigate that choice would show a deeper side of their relationship.