The combination of different characters' personalities. Iseei is known for his unique traits and Yasaka has her own charm. When they are paired in a fanfic, the way their personalities interact can be really engaging. For example, Iseei's determination and Yasaka's grace can create interesting storylines, like how they face challenges together in the supernatural world of High School DxD.
You can try looking on some popular fanfic websites like Fanfiction.net or Archive of Our Own. They usually have a wide range of fanfics, including those related to High School DxD. Just use the search function on the site and type in 'High School DxD Iseei x Yasaka' to see if there are any available.
One possible fanfiction could involve a crossover where the characters from Highschool DxD and Naruto meet in a unique world. Yasaka, being a powerful figure in the Highschool DxD universe, might interact with Naruto characters. For example, she could form an alliance with Naruto to fight against a common enemy that somehow threatens both of their worlds. Maybe a group of rogue ninjas from Naruto's world team up with some rogue devils from Highschool DxD.
Power is a common theme. Since the DxD universe has a complex power structure, many fanfictions explore how Yasaka navigates through it. For example, she might gain new powers or face challenges from more powerful beings.
In a Naruto and High School DxD fanfiction, there could be a scenario where the protagonists of both stories meet in a neutral dimension. Issei from High School DxD might be intrigued by Naruto's ninja skills and determination. Naruto, in turn, could be curious about Issei's Sacred Gear. They could start training together, sharing their knowledge and skills, and ultimately have to face a common enemy that threatens both their worlds.
Issei is often paired with Yasaka. Since he is a central character in Highschool DxD, many fanfiction writers like to create scenarios where he and Yasaka interact, sometimes romantically. Another character is Vali. His power and his own story arc make him an interesting counterpart to Yasaka in fanfiction.
One common theme is power - up. Naruto, already a powerful ninja, often gets new abilities or enhances his existing ones in the DxD world. Yasaka too might gain new powers or develop her skills further. Another theme is friendship. They form bonds with DxD characters and together face challenges.