In the realm of Sasuke DxD fanfictions, a common theme could be Sasuke's power growth. He starts with his existing ninja powers and then begins to incorporate DxD - related magic or abilities. Some fanfictions might show him as a lone wolf at first, but gradually warming up to the characters in DxD. Also, fanfictions could explore how his past traumas from the Naruto world affect his actions and decisions in the DxD universe.
Another interesting one could be 'Sasuke and the Occult Research Club'. This fanfiction might involve Sasuke getting involved with the club's activities. His knowledge of ninjutsu and his experience with the supernatural in his own world could bring a fresh perspective to the club's investigations and battles against the forces of evil in the DxD world.
One interesting crossover could be Sasuke in the Highschool DxD world using his Sharingan abilities during Rating Games. His combat skills would add a new dynamic to the battles in the DxD universe.
Issei would be a likely ally. He's the main character in Highschool DxD and is always looking for ways to become stronger. Sasuke's combat skills and knowledge could be very appealing to Issei. Also, Rias Gremory could be an ally. As a powerful devil and the leader of her peerage, she would recognize Sasuke's potential and value his skills in battles against common enemies.
One possible plot could be that Naruto and Sasuke, married, are somehow transported to the DxD world. They have to adapt to the new rules and powers there while also dealing with the reactions of the DxD characters to their relationship.
One idea could be that Naruto and Sasuke are transported to the DxD world and they have to learn the new rules and powers there. They might team up with some of the DxD characters to face a common enemy.
One possible interesting plot could be Sasuke arriving in the Highschool DxD world during a time of chaos. Maybe there's a new threat that neither the devils nor the other factions can handle easily. Sasuke's ninja skills and Sharingan would give him an edge in combat. He could form an alliance with Issei and his group, teaching them some ninja techniques in return for learning about the supernatural powers in their world.
There could be a fanfiction where Sasuke's Sharingan has an unexpected interaction with the supernatural elements in High School DxD. For example, his ability to see through illusions could be enhanced or altered by the magic in that world. This could lead to him having a different role in the battles and relationships within the High School DxD setting.
Well, one key plot element could be how Naruto and Sasuke adapt to the DxD universe. Maybe they have to deal with the different power systems like Sacred Gears. For example, Naruto's chakra control might interact in an interesting way with the power of a Sacred Gear. Sasuke, with his Sharingan, could have unique ways of seeing through the illusions or strategies in the DxD world.
You can try fanfiction websites like FanFiction.net or Archive of Our Own (AO3). On FanFiction.net, you can simply search for 'Sasuke DxD' in the search bar, and it will show you a list of relevant fanfictions. AO3 also has a great collection. Just make sure to read the tags and summaries to find the ones that suit your taste.