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.
The plot involves Rimuru's journey from being a rather weak (initially) slime to becoming a very powerful being. He meets many interesting characters. For example, there are the goblins he befriends early on. Rimuru helps them improve their living conditions. As the story progresses, he also has to deal with the threat of other more powerful monsters and even some human kingdoms that view his growing power with suspicion. In general, it's a story full of adventure, growth, and the forging of relationships in a magical and dangerous 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.