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.
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 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.
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.
There are likely some out there. You can start by checking popular fanfiction websites like FanFiction.net or Archive of Our Own. Just search for the key terms 'Naruto Sasuke married DxD crossover' and you might find some interesting reads.
Sasuke can fit in as a powerful outsider. His skills and abilities make him a force to be reckoned with. For example, his Sharingan can give him an edge in combat and in detecting hidden motives of characters in the DxD world.
Sasuke's character might become more open - minded. In the Highschool DxD world, he would be exposed to different cultures and ways of thinking. For instance, the concept of Devils and Angels having their own social structures would be new to him.
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.