One common theme is Asia's growth. Since she starts as a rather timid character in the original, fanfictions often focus on her becoming stronger. Another theme is her relationships, like her friendship with the main characters.
One interesting fan fiction could be about a new character joining the school and forming unique relationships with the existing characters. For example, a transfer student from a different supernatural realm who has different powers and a different view of the world in the DxD universe.
I really liked 'Asia's Unlikely Ally'. In this fanfiction, the OC is a transfer student with a special power. The way the author writes the interactions between Asia and the OC is really heartwarming. It also has some exciting action scenes that fit well with the overall High School DxD vibe.
Another way is to join fan communities dedicated to High School DxD on social media platforms like Reddit or Facebook. Fans often share their favorite fan fictions there, and you can also ask for recommendations. Sometimes, they might even link to some hidden gems that are not easily found on the big fan - fiction sites.
Their relationship might develop if they both find themselves in a situation where they are outcasts among their respective groups. They could bond over their feelings of not fully belonging and then decide to create their own place in the High School DxD universe. For example, starting a club for those who want to bridge the gap between angels, devils and fallen angels.
Another reason is that it fills in the gaps that the main series might leave. High School DxD has a rich world, but there are always untold stories. By creating an OC and pairing them with Asia, fans can create their own mini - adventures within that world. They can explore different power dynamics, friendships, and romances that add to the overall allure of the fanfiction.
One popular 'High School DxD Asia Fanfiction' is 'Asia's New Path'. It explores Asia's character development in new and interesting ways, showing her becoming more independent and powerful. Another is 'Asia and the Mysterious Relic' which has an exciting plot full of mystery and adventure.
One interesting plot idea could be that Raynare, who has had a change of heart after her initial actions, tries to make amends to Asia. Maybe she teams up with Asia to protect the school from a new, common enemy. This would show their growth and development as characters.
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.