You could also do a crossover with a steampunk world. Karna in a world of gears, steam - powered machines, and Victorian - style fashion would be quite interesting. His powers might be enhanced or modified by the technology of that world, and he would have to navigate the complex social hierarchies and political intrigue of the steampunk society.
Another possibility is to cross him with a fantasy world like Middle - earth from 'The Lord of the Rings'. Karna could be a great warrior in that world, perhaps facing off against orcs and other dark forces. His golden armor would stand out among the more medieval - style armors of Middle - earth, and his archery skills would be highly prized.
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.
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.
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.
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 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 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 idea could be a crossover with Ben 10. Rex's nanites and Ben's Omnitrix could lead to some interesting power clashes and team - ups. They could face a common enemy that requires both their unique abilities to defeat. For example, an alien - nanite hybrid villain.
One possible title could be 'The Crossover of Fate: Cu Chulainn's New Adventure'.
For 'fate crossover fanfiction', you could consider a crossover with 'One Piece'. The Servants might be pirates or marines, sailing the Grand Line. A 'Fate' and 'Fullmetal Alchemist' crossover would also be cool. Picture Servants trying to solve the mysteries of alchemy. There's also potential in a 'Fate' and 'My Hero Academia' crossover, with Servants having Quirks.