One possible scenario in a Ruka and Akatsuki fanfiction could be that they team up to protect Yuki from the pureblood vampires who have some devious plans. Ruka, with her loyalty to Kaname, might initially be hesitant to work with Akatsuki but realizes they have a common goal. Akatsuki, being a skilled vampire himself, could bring his combat abilities to the table while Ruka uses her knowledge of the pureblood society to strategize.
One common theme is friendship. Since Naruto is known for his ability to make friends, in fanfictions with Ruka, their friendship often plays a central role, like in 'The Bond of Naruto and Ruka' where they support each other through tough times.
They could be rivals in a fanfiction. Since Ruka is often focused on Kaname, and Akatsuki has his own loyalties and tasks within the vampire world, they might compete for resources or information that could benefit their individual causes. For example, if there's a limited supply of a special blood type that is valuable to vampires, they could both be vying for it for different reasons.
Love is a very common theme. Usually, it's about Ruka's love for Hayate or their mutual feelings. Another theme could be friendship. They might start as friends and then develop deeper feelings.
One possible plot could be that Hotaru and Ruka are a young couple who unexpectedly find out they are going to have a baby. They then have to navigate through the challenges of telling their families, preparing for the baby's arrival, and adjusting their own relationship to fit this new role of parents. For example, Hotaru might be worried about how to balance her career and motherhood, while Ruka could be concerned about providing for the family financially.
Overcoming obstacles is also a big theme. There could be external factors like family or society not approving of their relationship. And they have to fight against these to be together. For instance, in some stories, Ruka's career might be a hindrance to their relationship, but they find a way around it.