You can try fanfiction.net. It has a large collection of fan - created stories for various anime and manga, including High School DxD which Issei and Serafall are from. Just use the search function and type in 'issei x serafall' to find relevant fanfictions.
One popular one might be those that explore their unique relationship dynamic in the context of the High School DxD universe. For example, some fanfictions focus on how Issei's pervy nature interacts with Serafall's more flamboyant and powerful self. It could lead to some really interesting and comical situations, like when Issei accidentally makes a fool of himself in front of Serafall and she teases him mercilessly.
I haven't actually read a specific fanfiction with this plot. However, if such a fanfiction exists, it would likely involve a lot of creativity in terms of how the relationship between Issei and Serafall developed to that point. It could explore different aspects of their personalities and how they would handle the situation of a pregnancy, which might be quite different from the original storylines of their respective characters.
Romance is a big theme as well. Writers often focus on how the two characters from different backgrounds fall in love. It could involve Issei being in awe of Serafall's beauty and power at first, and then developing deeper feelings over time. There might also be elements of Serafall being initially dismissive but then growing to care for Issei.
The fanfiction could center around the cultural differences between Issei and Serafall. Since they come from different backgrounds in the supernatural realm, there could be humorous and touching moments as they try to blend their cultures, for example, in their wedding preparations. Serafall might introduce Issei to her unique magical traditions, and Issei could share his human - like values with her.
One possibility is that there would be a great deal of conflict within the story's world. Other characters might be shocked and try to interfere. For example, some might think it goes against the natural order or the rules of their society.
You can try fanfiction websites like Archive of Our Own (AO3) or FanFiction.net. AO3 often has a wide variety of well - categorized fanfics. Just use the search bar and type in 'Issei x Serafall' to see what's available.
Issei would likely become more confident and responsible. Marrying Serafall, a powerful figure, would force him to step up. He might learn new magical abilities or gain more respect in the supernatural hierarchy. For Serafall, she could become more in - touch with the human side of things through Issei. She may soften her more flamboyant and sometimes self - centered ways.
There might be a fanfiction where Issei accidentally stumbles into one of Serafall's secret training grounds. Serafall is initially angry but then realizes Issei's potential. She decides to train him in some unique devil magic. This leads to a series of comical and at the same time, character - building events as Issei tries to master the new skills while dealing with Serafall's unpredictable nature.
One common theme is the power dynamic. Since Serafall is a powerful character, exploring how an oc fits into that power structure can be interesting. For example, the oc could be a newbie who Serafall takes under her wing, or the oc could have some unique power that challenges Serafall's. Also, the theme of self - discovery often comes up. The oc might be on a journey to find out who they are and Serafall plays a role in that process. And of course, the theme of loyalty, whether it's the oc being loyal to Serafall or the other way around in different situations.