One popular trope is the 'protector' trope. Issei often takes on the role of protecting Asia from various threats in the fanfics, which plays on his character development in the original series as someone who wants to be strong and protect those he cares about.
Fanfiction.net might also be a good place. There are many fans who contribute their works there. However, the tagging system might not be as detailed as AO3, so you may need to do a bit more digging to find exactly what you're looking for in terms of Issei x Asia fanfics.
Protection is also a big theme. Asia is often depicted as a kind - hearted and somewhat naive character. So, Issei protecting her from various threats, whether it's other devils or supernatural beings, is quite common. This also serves to show his loyalty and love for her in the fanfictions.
One common theme is the exploration of their relationship in a more passionate or romantic way. Another could be the use of the lemon aspect to add a sense of sensuality. Also, their growth as characters within the context of these more adult - themed situations might be a theme.
You can try fanfiction websites like FanFiction.net or Archive of Our Own. On these platforms, you can search for 'Highschool DxD Issei x Asia' and find a variety of fanfictions. Some might be focused on their relationship development, while others could be adventure - based with their relationship as a side plot.
You can try looking on fanfiction websites like FanFiction.net or Archive of Our Own. Just search for 'Issei and Asia' in the search bar and filter through the results to find the lemon - themed ones. However, make sure to check the site's terms and conditions regarding such content.
Another interesting plotline is the exploration of Asia's past. Maybe she discovers some hidden powers or a connection to an ancient force. And Issei is there to support her. In these fanfics, we get to see a different side of Asia that wasn't fully explored in the original series. It often leads to some really exciting adventures and character growth for both of them.
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.
For a fanfiction with Issei and Asia married, a great plot could be that Asia discovers a new power within herself after getting married. This power could be related to their combined love and it attracts the attention of an ancient and evil force. Issei then has to train Asia to control this power while also protecting her from the approaching danger. Or perhaps they are chosen to represent their respective groups in a tournament that has high stakes for the future of their relationship and the groups they belong to.
A fanfiction about Issei and Asia's marriage in Highschool DxD could be really interesting. Since Issei has a strong sense of justice and Asia is a healer, they could form a powerful duo in the fanfiction world. They could start a new order in the supernatural community perhaps, with their combined powers and influence. Their marriage could also bring about a change in the power dynamics among the different factions in the story. Issei might use his influence as Asia's husband to broker peace between some of the warring groups, and Asia could use her healing abilities to help those affected by the ongoing conflicts.