There isn't a widely known and clearly defined 'love story' in the traditional sense between Kukulkan and Quetzalcoatl in a single unified narrative. However, both are important deities in Mesoamerican cultures. Kukulkan was a Mayan deity often associated with snakes and the sky. Quetzalcoatl was a significant Aztec deity, often depicted as a feathered serpent. They share similarities in their serpent - like forms and their connection to important aspects of the cosmos, but there is no common, well - established love story between them.
I'm not sure specifically as there could be many different takes on this. It might be about Percy being in a dangerous situation and Quetzalcoatl, a powerful figure from mythology, coming to his rescue in the context of a fan - created fictional story.
Naruto might be initially in awe of Tiamat's powers. Given his curious nature, he would likely try to understand how they work. He could attempt to adapt some of her power - using techniques into his own ninja fighting style.
The guild's acceptance of Tiamat would likely be a mixed bag. Some of the more adventurous and powerful members like Gildarts might be more open to having Tiamat around. They would see the potential benefits of having such a powerful ally. However, the younger or more timid members could be very hesitant. They might be worried about Tiamat's true intentions or its power getting out of control.
A great plot for a Fate Tiamat fanfic could be a scenario where Tiamat tries to adapt to the modern world within the Fate universe. She has to deal with human culture, technology, and the other strange things she never encountered before. This could lead to some comical as well as deep and meaningful interactions with the other characters.
You need to consider her origin as a primordial goddess. In a fanfic, show how her actions are influenced by her ancient nature. For example, she might be slow to understand modern concepts because she comes from a time long before. Her dialogue should also reflect her grandeur and power. Use complex language and refer to her ancient knowledge. Moreover, her relationships with other characters should be based on her fundamental characteristics, such as her sense of superiority as a goddess.
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.
In a fanfiction, Harry as a Quetzalcoatl Animagus could play a crucial role in a battle against Voldemort. His Animagus form could be resistant to certain dark spells, and he could use the large size and strength of the Quetzalcoatl to his advantage. Maybe he could also communicate with magical creatures from the Americas that are drawn to his new form, and they could help in the fight against the Death Eaters.
It's likely that the main characters from the original 'Highschool DxD' series would still play important roles. Maybe Issei Hyodo would be there, adapting to the new quetzalcoatl - influenced environment.
There's no way to know for sure who the author is without more information. It could be anyone who is a fan of the relevant source material (maybe Percy Jackson series) and has an interest in Quetzalcoatl from mythology.