A theme that often shows up is the exploration of their unique abilities. Issei's boosted gear and Kuroka's nekomata powers can be combined in interesting ways in fanfiction. There can also be a theme of family, as Kuroka has her sister, and Issei has his group of friends who are like family, and how their relationship fits into that dynamic.
Some popular themes in Issei and Kuroka fanfiction could be adventure. They might go on quests together in the supernatural world of Highschool DxD. Another theme could be relationship development, like how they gradually fall in love and overcome the differences between a human - turned - devil like Issei and a nekomata like Kuroka.
One common theme is power - up. Issei is always looking to get stronger, and Kuroka can be involved in his training or in battles where he gains new powers. Also, the theme of redemption can play a part. Kuroka has her past misdeeds, and through her relationship with Issei, she might find a path to redemption.
One interesting plot idea could be that Kuroka and Issei have to team up to solve a mystery in the Underworld. Maybe there are strange disappearances of some low - level demons, and they suspect a powerful, hidden force is behind it. As they work together, their relationship deepens from just comrades to something more romantic.
One possible plot could be that they decide to have a traditional wedding in the Underworld. All their friends and allies from the different factions in Highschool DxD come together. Koneko might be initially hesitant but then fully supports her sister Kuroka. Issei would be nervous but excited, and there could be some comical moments as he tries to follow all the wedding customs.
One plot idea could be that Issei and Kuroka are forced to team up for a school project. During this time, they start to really understand each other's personalities beyond their initial impressions. Issei discovers Kuroka's softer side that she usually hides, and Kuroka sees Issei's determination and loyalty. This could lead to the start of their romantic feelings.
Rias would definitely be a guest. As Issei's former master and a close friend, she would be there to support him. Akeno would also come, being Rias' queen. Koneko, Kuroka's sister, would be there of course. Xenovia and Irina might also be invited as they are friends with Issei. From the devil side, Sirzechs and Grayfia could be present as important figures in the devil society.
Power - up themes are also common. Since Issei has his own power progression in the original series, fanfictions might explore how he and Asia grow in power together. Maybe they train together or discover new abilities as a team. And of course, there are the slice - of - life themes. These stories show their daily lives at school or in the supernatural world, dealing with normal and not - so - normal problems.
Writers often portray their relationship as a slow - burn romance. At first, there might be attraction due to their physical appearances and powers. Then, as they interact more, they start to understand each other on a deeper level.
To write their character development well, give them common goals. Let's say they both want to improve the relations between different factions in the DxD world. As they strive for this goal, they learn from each other's strengths and weaknesses. Issei can learn Kuroka's stealth and cunning, while Kuroka can learn Issei's determination and ability to inspire others. This mutual learning process will enhance their character development.
Well, Kuroka is a very alluring character in the Highschool DxD universe. Her feline features and her somewhat devious nature make her interesting. Fanfiction about her often explores her relationships with other characters in more depth. For example, her relationship with her sister, Shirone. Writers can also play around with her powers and create exciting new storylines. In addition, fans like to see different versions of her, like a more heroic Kuroka or a Kuroka in different settings.
Another theme might be the comical aspect of the 'blue balling' situation. Fanfiction writers might use it to create humorous scenarios, like Issei's reactions and how he tries to overcome this particular aspect in his life in a comical manner. This can add a light - hearted touch to the overall story.
Some fanfictions might focus on the magical or supernatural aspects of the chastity belt. Maybe it has some special powers or curses attached to it that Issei has to figure out. This could lead to adventures where he has to search for a way to break the spell of the belt while also dealing with the normal challenges in the Highschool DxD world like fighting devils and angels.