One reason could be a miscommunication. Maybe a misunderstanding between them leads Arturia to think Shirou has done something unforgivable, so she betrays him.
It could be due to external forces. For example, if there is some powerful entity that manipulates Arturia's mind or forces her to act against Shirou. Another possibility is that Arturia's own sense of duty gets twisted in the fanfic. Maybe she believes that betraying Shirou is for the greater good in some convoluted way, perhaps to save her kingdom in a different timeline or something similar.
I'm not sure about specific 'arturia betrays shirou' fanfics. You could try searching on popular fanfic websites like Archive of Our Own or FanFiction.net.
One possibility is that it could explore their complex relationship. Since Shirou becomes Archer, he has a different perspective. He might be more jaded and less idealistic. With Arturia, their relationship could be filled with tension. Archer Shirou might initially push Arturia away, seeing his past self's naivete in her. But eventually, they could come to understand each other better. Their shared experiences in different timelines could also be a big part of the story, like how they both fought in the Holy Grail Wars but in different ways.
Well, the theme of acceptance is quite common. They come from different backgrounds and have different personalities. Fanfics show how they accept each other's flaws and differences. Another theme is the power of hope. Their relationship gives them hope in a world full of chaos and danger. They see each other as a source of hope and motivation. And there's also the theme of the past haunting the present. Their past experiences, both good and bad, influence their relationship in the present, and fanfics often explore how they deal with these ghosts from the past.
One possible plot could be that Irisviel summons Archer Shirou in a world where Arturia already exists. Maybe Irisviel has a specific plan to use Archer's abilities to help Arturia in some great battle or quest. Archer might initially be reluctant to work with Arturia due to their past differences, but as the story progresses, they start to understand each other better and form a strong alliance.
One possible reason could be power. Maybe in the fanfic, there's a new source of power that Tsunade wants desperately and Naruto stands in her way to obtain it. So she betrays him to get to that power.
Another aspect could be related to her past or origin. If she has some hidden memories or a secret agenda tied to her existence as the Fem Kyubii, it could lead her to betray Naruto. For example, if there are some dark forces that are controlling or influencing her from her past, she may be forced to turn against Naruto against her will in a sense.
The concept of time and fate is important. Since they exist in different timelines and yet are brought together in this fanfic, it can add a layer of mystery and destiny to their relationship. For example, Archer Shirou might be trying to change certain aspects of his past related to Arturia. Their relationship could also be influenced by the power of the Holy Grail. Maybe it has some lingering effect on them that either draws them together or creates obstacles. Additionally, the exploration of their unspoken feelings, especially Archer Shirou's repressed emotions towards Arturia, can be a major element in the fanfic.
One popular shirou x arturia fanfic is 'Avalon's Dream'. It beautifully weaves their relationship with elements from the Fate series lore. The story delves deep into their emotions and the challenges they face together. Another great one is 'Swordbound Hearts'. It focuses on their journey after the events of the main series, exploring how their bond grows stronger over time. There's also 'Fate's Bond' which is known for its detailed characterizations of both Shirou and Arturia, making their relationship feel very real and engaging.
Another theme is the idea of destiny. Often in these fanfictions, their being summoned together is seen as part of a greater destiny. For example, in some stories, they are meant to save a certain world or defeat a powerful enemy. It gives a sense of purpose to their relationship. Also, the theme of self - discovery is quite prevalent. As they face challenges together, they learn more about themselves and their own powers. This is especially true for Shirou as he often discovers new aspects of his abilities when paired with Arturia.
One possible reason could be that Naruto discovers some dark secrets within Konoha. Maybe he finds out that the village elders have been using him all along, not truly caring about his well - being but only his power as a Jinchuriki. This realization could be so shocking that he decides to turn against the village.