Love and friendship are common themes. Their relationship often starts as a friendship that gradually evolves into something more. For example, in many fanfics, they support each other through tough times, which deepens their bond.
One popular trope is the 'protector' trope. Issei often takes on the role of protecting Asia from various threats in the fanfics, which plays on his character development in the original series as someone who wants to be strong and protect those he cares about.
I really like the fanfic 'Koneko's Heart for Issei'. In this one, it delves deep into Koneko's emotions towards Issei. It starts with her being a bit reserved but as the story progresses, her true feelings start to surface. Another interesting fanfic is 'Issei and Koneko: United'. This fanfic shows how they work together to face challenges in the Highschool DxD world, and their relationship blossoms as they rely on each other.
Fanfiction.net might also be a good place. There are many fans who contribute their works there. However, the tagging system might not be as detailed as AO3, so you may need to do a bit more digging to find exactly what you're looking for in terms of Issei x Asia fanfics.
A common theme could be the development of their relationship. Since it's a fan - created pairing, many fanfics might focus on how they get to know each other better, overcome differences, and grow closer.
One great fanfic could be 'Issei and Gasper's Unlikely Bond'. It delves into how their friendship develops in a unique way, with some exciting adventures in the Highschool DxD universe. They face challenges together and support each other, which is really heart - warming.
One common theme is the exploration of their relationship dynamics. Given that Ophis is a very powerful being and Issei has his own growth journey in the series, fanfics often delve into how they interact and influence each other's development.
Love against all odds is also a frequent theme. Given their different positions and the nature of the High School DxD world, their love might be seen as something that goes against the norm. So, many fanfics will center around how they fight for their love in the face of various obstacles, whether it's social hierarchies or threats from other characters. It makes for a very engaging read as the story unfolds.
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.
One reason for the popularity could be the appeal of the characters Issei and Ravel themselves. Fans who love these characters might be interested in seeing their relationship take on a more romantic or even sensual turn in a lemon fanfic. Also, the lack of such explicit exploration in the official content might drive fans to seek it out in fan - made works. Additionally, the creativity of the fanfic authors in developing their relationship and the scenarios they create can contribute to its popularity.