Well, Kirei and Gilgamesh make for an interesting pairing in fanfiction. Their complex personalities can lead to some really intense and thought - provoking stories. Kirei's inner conflict and Gilgamesh's haughty nature can create a lot of drama and tension.
In this type of fanfiction, the 'fated encounter' trope is quite popular. Writers like to show how Kirei and Gilgamesh's paths cross in a way that seems almost predestined. It gives a sense of grandiosity to their relationship. Additionally, the 'clash of ideals' trope is often used. Gilgamesh's self - centered and somewhat hedonistic ideals might clash with Kirei's more conflicted moral compass, and this can create a lot of interesting plotlines as they either try to reconcile or go their separate ways.
I'm not aware of its popularity without more information. There are countless visual novels out there, and its popularity would depend on various factors like the quality of the story, the art, and the marketing.
Another idea is that Kiritsugu summons Shirou because he foresees a future threat that only Shirou can prevent. This could involve time - traveling elements. Shirou might have to go back in time to change certain events. During this process, he and Kiritsugu would have to work together to figure out how to manipulate time without causing major disruptions.
I'm not sure specifically as there could be many interpretations. It might be a visual novel related to a beautiful wife ('tsuma ga kirei' can mean'my wife is beautiful' in Japanese). Maybe it's a story that revolves around the relationship with a beautiful female character, perhaps including elements like love, family, or daily life in a fictional setting.
One common theme is redemption. Given Kiritsugu's complex past, many fanfics explore his journey towards redeeming himself in the context of the Beast Lair.
One common theme is the clash of different moral codes. Kiritsugu has his own set of beliefs, and when he is crossed over into other worlds, like a world of superheroes or a magical realm, his way of thinking often clashes with the local characters. For example, in a crossover with a world of knights, his willingness to use underhanded tactics for the greater good might be at odds with the knights' chivalry.
One great kiritsugu emiya crossover fanfiction is 'Kiritsugu in the World of Harry Potter'. It imagines Kiritsugu being summoned to the wizarding world. His skills and unique moral code create really interesting interactions with the characters there. Another one could be 'Kiritsugu and the Avengers Crossover'. Here, he teams up with the Avengers and his strategic mind helps them in battles in a very different way compared to their usual approach.
In such a fanfic, it could focus on the battles they fight together. With Archer's long - range capabilities, Emiya's versatility, and Saber's powerful swordsmanship, they could be an unbeatable team. Kiritsugu might be the strategist behind their every move. The story could also delve into their pasts. Saber's past as a king, Archer's as a hero, and how those pasts influence their actions in the present. It could be a really exciting story full of action and drama.