You can try fanfic websites like Fanfiction.net or Archive of Our Own. On these platforms, you can search for 'Highschool DxD Asia and Issei' in the search bar, and it will show you a list of relevant fanfics. Just make sure to read the tags and summaries to find the ones that suit your taste.
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.
I'm not sure if there is a specific fanfic exactly like that. Fanfics can vary widely, and while there might be some that involve Issei's sister in the Highschool DxD universe, a 'lemon' themed one might be harder to find or might not be in line with the general community guidelines. It's important to note that 'lemon' usually implies adult - themed content which is often restricted in many fanfic platforms.
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.
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.
Another place to look is Reddit. There are subreddits dedicated to Highschool DxD fanfiction where users often share their favorite stories or even write their own. Just search for relevant subreddits and start exploring. Some fan-made forums related to the series might also be a good source.
One plot idea could be Asia and Gasper teaming up to solve a mystery within the school. Maybe there are strange disappearances and they use their unique abilities to figure out what's going on. Another idea is a story where they go on a journey to find a magical artifact that can help protect their friends. And a third could be a slice - of - life story where they help each other deal with personal problems like Gasper's shyness and Asia's insecurities about her new life in the supernatural world.
For Asia, her character could be developed by exploring her past more deeply. Maybe she could have flashbacks to her life before joining the supernatural world, which would help explain her kindness and her eagerness to help others. As for Gasper, he could be made to overcome his extreme shyness. For example, by being put in situations where he has to speak up to protect Asia or their friends. This would show his growth and development.