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.
Chapter 80, "Princess Wen, Your Lunch is Escaping!", is the clearest early answer because it turns Set Meal's situation into a concrete plot event. The chapter turns that into something concrete by moving from Jiang Yan and Wen Jiaojiao, one so smug he was almost ascending to...
Chapter 70, "Irresistible Stunning Charm", is the clearest early answer because it turns Aunt Wang's situation into a concrete plot event. A useful checkpoint is the way the scene starts with So, under the curious gazes of everyone, Jiang Yan strutted up with...
Chapter 334, "Hanqiong's Seven Divisions, Moao Mountain, Beishuo Revisited 4", is the clearest early answer because it turns Zhao Shen's situation into a concrete plot event. The setup is Lu Dong looked at the thirty members of the Steel Mane Team and..., but the pressure keeps building through Surprisingly, there were seven, and a look of astonishment appeared....
Chapter 302, "Signs of Defeat Emerge, Snow Bamboo's Trump Card", is the clearest early answer because it turns Longyou Army's situation into a concrete plot event. The chapter turns that into something concrete by moving from The defenders kept falling, even those at the Cold Resistance Level... to Your Xia City is as solid as gold, is it possible that my Long....
Chapter 22, "Number 7203", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event. Early on, Hearing this, Su Yinwan looked at Shang Shaojing with a face full of.... By the end, Jiang Li's righteousness temporarily shut Jiang Yan up, but after a....
Chapter 350, "Methods to Enhance the Peak of Cold Resistance, Moao's 10th Town, Beishuo Yang Fa, Nightmare, East Ridge Lu Yun", is the clearest early answer because it turns Tiger Power Pill's situation into a concrete plot event. A useful checkpoint is the way the scene starts with The first kind of Tiger Power Pill is unclear, but just by hearing... and follows through with Believe me, there will be future opportunities for us to interact!"....
Chapter 168, "Gu Family in the Capital", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event. The chapter gives readers a real answer by opening on Su Yinwan spoke those words with a pitiable expression, then lowered...
Chapter 306, "Helplessness, Shifting Loyalties, Total Annihilation and New Territory (Part 6)", is the clearest early answer because it turns Bai Wuting's situation into a concrete plot event. Early on, Next to him, Chen Yingyuan couldn't help but speak.. By the end, After saying this, he frowned and promptly ordered: "Li Xuanling and....
Chapter 50, "Girls Help Girls!", is the clearest early answer because it turns Are's situation into a concrete plot event. The chapter gives readers a real answer by opening on [666, if good scores determine rankings, is Flash Change's production...
Chapter 166, "Daniu Gets Jealous!", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event.