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.
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.
Perhaps the plot could center around the baby having some sort of special power that is not immediately understood. This power could attract the attention of both good and bad forces. Issei and Kuroka would have to team up with their friends and allies to figure out what this power is and how to control it. Along the way, they would have to deal with the normal difficulties of raising a child, like teaching it right from wrong and how to use its powers responsibly. This would be a great opportunity to show the growth of Issei and Kuroka as parents and as individuals in the Highschool DxD universe.
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 fanfiction idea could be that Issei and Kuroka's relationship develops in a way that they decide to start a family. Kuroka, with her feline nature, might be initially hesitant about motherhood. But Issei's determination and love for her could change her mind. They could face challenges from the supernatural world around them, like other factions being against their union due to Kuroka's past. However, their love for each other and their unborn child could be the driving force to overcome these obstacles.
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.
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 Rias and Issei wedding fanfiction could be full of grandeur. There would be various characters from the Highschool DxD universe in attendance. The wedding might take place in a magnificent location, perhaps a grand cathedral-like place in the Underworld. Rias would look stunning in a traditional yet unique wedding dress, and Issei would be in a sharp suit. Their friends would give heartwarming speeches, and there would be a sense of celebration not just for the couple but for the unity of different factions in the story.
A simple plot could be that they compete in a school - wide tournament. Koneko's pride as a Nekoshou and Issei's determination to prove himself make for an interesting dynamic. They both face tough opponents and along the way, they offer each other advice and support, gradually becoming closer friends.
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.