First, you need to understand the characteristics of each universe. For Highschool DxD, know the different devil clans, their powers and hierarchies. In Naruto, study the ninja villages, jutsus and the chakra system. For Tiamat, research her origin and abilities. Then, create a plot that can bring these elements together. It could be a quest to save a world, or a tournament where characters from all three universes participate.
One possible fanfic could involve a scenario where the characters from Highschool DxD and Naruto end up in the same universe as Tiamat. Maybe the devils from Highschool DxD are intrigued by the unique powers in the Naruto world and they team up to face Tiamat. They could combine their different combat skills, like the devil's magic and Naruto's ninjutsu, to fight this powerful entity.
One interesting 'Highschool DxD Tiamat' fanfiction could be the ones that explore Tiamat's character in the context of the Highschool DxD universe. For example, some fanfictions might focus on her relationships with other characters like Issei. They could show how she adapts to the school environment and the chaos that often ensues in the Highschool DxD world. Another aspect could be fanfictions that delve into her powers and how they are perceived and used within the story. Maybe she uses her powers to protect the school or to help a particular character achieve their goals.
Tiamat could be developed further by exploring her relationships with other characters. For instance, if she starts to mentor a young character, it would show a different side of her. She could teach them about her powers and the history of her kind.
Another way is to join Highschool DxD - related forums or communities. People in these groups often share and recommend fanfictions. They might even have a dedicated section for 'Tiamat' - related stories. You can ask around in these communities about any good 'Highschool DxD Tiamat' fanfictions they know of, and they'll be more than happy to point you in the right direction.
Yes, there are quite a few. You can start by looking on fanfiction websites like Archive of Our Own or FanFiction.net. Some might focus on their unique relationship dynamic, exploring how Issei's character develops in relation to Tiamat.
One common theme is power. Since Tiamat is a very powerful being in the 'Highschool DxD' world, fanfictions often explore how Issei's power grows or is affected by his relationship with her.
One plot idea could be Tiamat adjusting to the modern world in the Highschool DxD universe. Maybe she has to deal with the different social norms and the unique magic systems there.
Naruto would probably be very friendly and curious. He'd be excited to meet all the different types of beings in the Highschool DxD world. He might start by trying to make friends with Issei, sharing his stories about his own battles and adventures in his world.
One idea could be a crossover where Naruto characters end up in the world of Highschool DxD. For example, Naruto himself could be transferred to the school in Highschool DxD. He might use his ninja skills in the battles against the devils and fallen angels there. His shadow clone jutsu could really surprise the characters in the Highschool DxD world.