I'm sorry I can't directly recommend specific fanfics as it depends on various platforms and the constantly evolving nature of fan - created content. However, you can start by looking on popular fanfic websites like Archive of Our Own (AO3). Just search for 'Chu Chulainn Fate Caster SI' and you'll likely find a good number of them. You can then read the summaries and reviews to determine which ones might be good for you.
One common theme is the exploration of Chu Chulainn's power in new and different scenarios. Since he is a powerful and complex character in the Fate universe, fanfic authors often like to test his limits. Another theme could be the relationship between the self - insert (SI) character and Chu Chulainn. It might involve the SI character learning from him, or a romantic relationship developing. Also, there could be themes related to the overall Fate lore, like the Holy Grail War, and how the SI and Chu Chulainn interact within that context.
Cu Chulainn and Diarmuid in Fate fanfic can be explored in many ways. One aspect is their relationship with other characters. For example, how they interact with their masters. Cu Chulainn's relationship with his master might be more straightforward, while Diarmuid's could be more complex due to his sense of honor. Another point is their backstories. Fanfic writers often delve deeper into their pasts, which can add more depth to their characters. Their roles in the overall Fate universe within the fanfic can also vary greatly, from being central heroes to supporting characters in a larger story.
In Fate fanfic, Cu Chulainn can be the wildcard. His unpredictable nature, especially in his berserker form, can introduce an element of chaos into the story. He might break the rules or go against the established order to achieve his goals. Diarmuid, in contrast, is more likely to uphold the rules. His role could be to keep Cu Chulainn in check or to find a middle ground between following the rules and achieving victory. Their combined presence in a storyline can create a dynamic where there is a constant push and pull between chaos and order, brute force and diplomacy.
The crossover aspect needs to be carefully crafted. Decide which other elements from different universes or stories are being crossed over with Fate. It could be a different mythological setting or another anime world. Explain how the rules of the Fate universe interact with those of the crossed - over element. For instance, if it's a crossover with a magic - based world, how does Cu Chulainn's own magic system blend in? Also, the plot should revolve around a central conflict or goal. Maybe Cu Chulainn has to save both his original Fate world and the crossed - over world from a common enemy.
I'm not sure specifically what 'cywren caster fanfic' is about as it could be a very niche or personal creation. It might be a fan - made story related to a character named Caster by an author named Cywren.
One common theme is his struggle with his identity. Since he has a complex backstory, many fanfics explore how he comes to terms with who he is. Another theme is his relationships, whether it be with other servants or with the masters. And of course, his magical exploits are often a central theme as well.
One common theme could be the struggle of the self - inserted character (SI) in the complex world of Fate Greek. They often have to deal with the pre - existing power structures and mythological beings. Another theme might be self - discovery, as the SI figures out their place and role in this new and often dangerous setting.
She would face a lot of challenges. Firstly, cultural differences. The Fate Stay world has its own cultural and mythological backgrounds that are different from the Harry Potter universe. She would need to understand and respect these differences to function well. Secondly, power - wise, there are many powerful Servants in Fate Stay. She would have to find a way to compete and hold her own. This might mean she has to constantly train and improve her skills. Also, in terms of her identity, she has to balance being Harry Potter with being a Caster in Fate Stay. If she leans too much towards her Harry Potter side, she might not fit in, but if she abandons it too much, she loses what makes her character interesting.