One idea could be to cross Karna with a modern - day superhero universe. Imagine Karna's powers and his noble character in a world full of caped crusaders. He could be a sort of lone wolf hero, respected for his strength but also feared for his unwavering sense of justice.
Finally, have a satisfying resolution. Whether it's Karna achieving a great victory or making a self - sacrificing move for the greater good of the crossover world, the ending should tie up loose ends. Don't forget to include some twists and turns along the way to keep the reader engaged. Maybe Karna discovers a hidden power within himself that is only unlocked in the crossover world.
It's hard to name the single 'best' author, but there are some well - known ones. Some authors who have a great understanding of Karna's character and the Fate universe often create engaging fanfictions. For example, those who have been writing in the Fate fanfiction community for a long time and have received a lot of positive feedback from readers.
One interesting Karna Fate fanfiction could be about Karna's journey in an alternate universe where he forms an unexpected alliance. Maybe he teams up with characters he never interacted with in the original story, like some of the mages from a different faction. This could lead to all kinds of new adventures and character developments.
One common theme is power comparison. Since Karna is a very powerful character, in crossovers, his powers are often compared to those of the characters in the other universe. For example, in a crossover with the Marvel universe, his archery skills might be compared to Hawkeye's.
One popular 'Karna Crossover Fanfiction' is 'Karna in the World of Naruto'. It imagines Karna's character in the Naruto universe, with his powerful abilities creating interesting storylines. Another one could be 'Karna Meets the Avengers', where his skills and noble nature contrast and interact with the superheroes.
One idea could be to explore how Hercules' incredible strength affects the Fate universe. Maybe he is summoned as a Servant in a Grail War and his brute force challenges the traditional battle strategies of other Servants. His Twelve Labors could also be incorporated in some way, like each Labor being a trial he has to face within the context of the Fate story.
A third idea is a crossover where the characters from both universes have to team up to face a common enemy. Maybe an ancient evil that threatens both the world of Fate and the DxD world. They would need to combine their powers and strategies, like the devils using their demonic powers along with the Servants' unique abilities.
One great 'Fate Servant Crossover Fanfiction' is 'Fate/Grand Order and the World of Harry Potter'. It combines the magic of the Harry Potter universe with the unique Servants from Fate. The story often has interesting battles where Servants use their powers in the Hogwarts setting. Another one could be the crossovers with the Naruto world. Here, Servants might face off against ninja techniques, creating a whole new dynamic of power clashes.
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.