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.
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.
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.
Chapter 263, "Offering Advice, An Elder Brother Like a Father", is the clearest early answer because it turns Xiao Hen's situation into a concrete plot event. The setup is Big sis, second sis, I brought soup." "Coming!" Outside the main..., but the pressure keeps building through Didn't I tell mom to stop making it?.
Chapter 243, "Hanqiong's Great Battle, Realization After the Fact", is the clearest early answer because the buildup finally cashes out in a real clash. It opens with Longyou Year 48, April 17 On the rooftop of the main building at... and closes on Are they trying to monitor and restrict me to the Hanqiong Station,.... The key payoff is that the danger stops being abstract and the conflict around trade and resources becomes immediate, forcing the surrounding cast to react under pressure.
Chapter 350, "Methods to Improve the Peak of Cold Resistance, Moao's 10th Town, Beishuo's Yang Fa, Nightmare, Lu Yun of East Ridge", is the clearest early answer because it turns Great Xia's situation into a concrete plot event. The setup is Especially like the Nine Towns, not only surviving but also..., but the pressure keeps building through No matter how smart Xia Hong is, wisdom is limited..
Chapter 253, "Short on Meat, Short on Manpower, Someone Delivers (Part 2)", is the clearest early answer because it turns Xia Chuan's situation into a concrete plot event. The chapter turns that into something concrete by moving from Beehive 129, last night Hanqiong's 1259, with the newly added... to Our stockpile of beast meat is considerable; if the population....
Chapter 318, "Transformed Li Xuanling, Reform, 9th-Class Viscount (Part 3)", is the clearest early answer because it turns Xia Chuan's situation into a concrete plot event. It opens with Scout Colonel Liu Yuan of the Cloud Dragon Army Reconnaissance Camp,... and closes on Contribution points and silk weren't considered significant, the main....
Chapter 271, "Eliminated, Breakthrough Surge, Green Lotus Grass (2)", is the clearest early answer because Xiang Liang finally turns buildup into a real advancement. The chapter turns that into something concrete by moving from Xiang Ping felt dizzy as he stepped off the stage. to Crucially, this is only in Xia City; including Mirror Valley, Wuyuan,....
Chapter 272, "Disappointment and Surprise, Surge in Cold Resistance Level Participants", is the clearest early answer because Redwood Ridge finally turns buildup into a real advancement. Early on, Second Year of Great Xia, June 2, Late Night Xia City is still.... By the end, Including Li Baihe and the others, Luo Ming naturally dared not voice....
Chapter 264, "Supplies Delivered, News from Long Mountain (3)", is the clearest early answer because it turns Xia Chuan's situation into a concrete plot event. The setup is He has been stationed in Longyou for over twenty days now, and he..., but the pressure keeps building through Taking advantage of this period without battles, he dispatched over a....
Chapter 333, "Silver Mine from the Heavens, Yu Yao and Yu Sheng, No Need to Use a Bull Knife to Kill a Chicken [10,000-Word Mega Chapter Continued]", is the clearest early answer because the violence around Xia Hong becomes final instead of hypothetical. It opens with Everyone, thank you for waiting!" Xia Hong's voice came from the... and closes on Meng Ying and the others were filled with excitement, having secured....
Chapter 331, "Zhu Shun, Huang Tianxi, the Third Cold Yuan Banquet of Great Xia, and Xia Yuzong", is the clearest early answer because it turns Huang Tianxi's situation into a concrete plot event. The setup is Upon hearing the excited sound of his nephew running towards the..., but the pressure keeps building through The gate opened, and a small fist suddenly punched straight out,....