Kaiju novels often portray kaiju as gigantic and powerful beings. They are usually depicted as forces of nature that are difficult to control or defeat. For example, in 'Godzilla' novels, Godzilla is shown as a nearly indestructible creature with its atomic breath and massive size.
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.
One popular 'kaiju x kaiju fanfic' story could be about Godzilla and Mothra forming an alliance in a unique way. Maybe they team up against a new, more powerful enemy that threatens their world.
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.