Well, 'Time - Twisted Shirou' is another good one. In this fanfiction, Shirou's time travel is caused by a strange magic spell gone wrong. As he travels through time, he discovers secrets about the Grail War and his own powers that he never knew before. It has a lot of action and some really cool plot twists.
One common theme is redemption. Since Shirou can go back in time, he often tries to fix his past mistakes or prevent tragedies that occurred in the original story, seeking redemption for himself and others.
One possible fanfiction could be where Rin and Shirou's kiss is a moment of realization. Maybe they've been through a tough battle together and in the heat of the moment, their feelings overflow and they share a kiss. It's a way to show their deep connection and the growth of their relationship.
In these crossovers, the theme of growth is quite common. Shirou starts as his normal self but through his experiences in the new world, he grows as a character. For example, in a crossover with the 'Naruto' world, he may learn new combat techniques and also gain more understanding of different kinds of power. Another theme is the exploration of new power dynamics. How his power compares to the powers in the new world, like if he's in the 'Bleach' universe, how his abilities stack up against Soul Reapers. And the theme of moral dilemmas also exists. He may face situations where the moral code of the new world challenges his own.
In many fanfictions, Shirou and Rin's kiss is a highly anticipated and emotional moment. It often symbolizes a deepening of their relationship. Some fanfictions might build up to this kiss through a series of adventures and self - discoveries. For example, they could be in the middle of a battle, and in a moment of respite, their feelings for each other overflow, leading to the kiss.
Another angle in fanfiction could be the exploration of their character growth through their relationship. Shirou, who starts off with a rather idealistic view of heroism, might learn more about the reality of sacrifice and love from Sakura. Sakura, on the other hand, could gain more confidence and a sense of self - worth through Shirou's unwavering support. This mutual growth can be a fascinating aspect to explore in fanfiction.
There are also fanfictions that explore Sakura's darker side in relation to Shirou. Given her connection to the Grail and the secrets she holds, she could be a more complex and dangerous figure. However, Shirou's love for her might be the thing that tames or redeems her in these stories.
One great 'Fate Stay Night Shirou Crossover Fanfiction' is 'Shirou in the World of Harry Potter'. It combines the magic of the Harry Potter universe with Shirou's skills and character traits. Another interesting one is 'Shirou and the Avengers Crossover'. Here, Shirou gets involved with the Avengers and has to use his abilities in a completely different context. And there's also 'Shirou in the Land of Westeros' which is a really unique take as it mixes the 'Fate Stay Night' world with that of 'Game of Thrones' where Shirou has to navigate the complex politics and battles of Westeros.
Perhaps in this fanfiction, Shirou discovers a hidden power within himself during the 4th War. This power might be related to his ideals of being a hero. He could use this power to try and stop the more malicious servants from causing too much destruction. For example, he might be able to counter Gilgamesh's arrogance with his newfound strength.
Love is a very common theme. Their relationship, which was complex in the original work, is often explored more deeply in fanfiction, showing different aspects of their love for each other.
It could be Gilgamesh. Gilgamesh is extremely powerful and has a very different view of the world compared to Shirou. Their ideals would clash, with Gilgamesh seeing Shirou as a weakling with naive dreams of heroism, while Shirou would see Gilgamesh as a tyrant who needs to be stopped.