An OC (Original Character) in Fate Apocrypha fanfiction can bring fresh perspectives. It allows fans to explore uncharted territories within the Fate Apocrypha universe. For example, an OC could be a new Servant with a unique backstory and abilities, which might interact with the existing characters in interesting ways. Fanfiction writers often use OCs to fill in the gaps they feel exist in the original story or to create their own unique story arcs that still fit within the overall theme and lore of Fate Apocrypha.
Another concept is a more comical crossover. Shirou tries to adapt to the different rules and magic in the Fate Apocrypha world. He constantly gets into funny situations as he misunderstands the local customs and magic systems. For example, he tries to cook using Apocrypha - style magic ingredients and chaos ensues.
One popular trope is the 'hidden power' trope. The OC might start off as an unassuming character but gradually reveals a powerful and unique ability that changes the course of the Holy Grail War. Another is the 'love interest' trope, where the OC becomes romantically involved with one of the canon characters, adding a new layer of drama to the story.
The ending is also different. The anime's ending might seem a bit more abrupt and less conclusive in some ways compared to the novel. The novel goes into more detail about the aftermath of the war and how it affects the world and the remaining characters. It ties up loose ends more effectively, while the anime leaves some viewers with a sense of incompleteness.
Perhaps a plot where the Holy Grail War from Fate Stay Night gets disrupted by the arrival of some key players from Fate Apocrypha. This could lead to new alliances and rivalries. Like Shirou from Fate Stay Night having to team up with a character from Fate Apocrypha, say Sieg, to face a common enemy that is after the power of the Grail in both their worlds.
One common theme is the clash of their personalities. Gilgamesh is proud and haughty, while Mordred is rebellious. Their interactions often involve power struggles and attempts to outwit each other.
Sieg is initially naive. But he has a strong will. He is also very adaptable. As he is thrust into the world of the Holy Grail War, he quickly learns to survive and fight.
Rin Tohsaka from Fate Stay Night and Astolfo from Fate Apocrypha. Rin is a serious and intelligent mage, while Astolfo is a carefree and somewhat eccentric Servant. The combination of Rin's no - nonsense attitude and Astolfo's wacky behavior would make for some really interesting and humorous exchanges.
Love and friendship are also common themes. An OC could develop a deep friendship or even fall in love with Mordred. This relationship can be complicated by Mordred's status as a Servant and the events of the Holy Grail War. It can also be a source of character development for both the OC and Mordred. For instance, Mordred might learn about different forms of love and loyalty through her relationship with the OC, while the OC might grow in courage and determination as they get closer to Mordred.