One idea could be a story about a Dark Eldar wych who is trying to rise in the ranks within her cult. She could face challenges from rival wychs and have to prove her worth in the gladiatorial arenas of Commorragh.
One idea could be that K has some unique jutsu that Naruto wants to learn. So he teams up with K to go on a journey to master this new type of jutsu. This could lead to a lot of fun interactions as Naruto's energetic and determined personality clashes and also meshes with K's own characteristics.
One idea could be a crossover with Star Wars. Imagine the Imperium of Man encountering the Galactic Empire. The two totalitarian regimes could have some intense ideological battles and epic space battles as well.
One interesting idea could be the combination of Naruto's ninja abilities with the high - tech and grimdark elements of Warhammer 40k. For example, Naruto's shadow clone jutsu might be used in a unique way on the battlefields of the 41st millennium. Maybe the clones could be used to infiltrate enemy lines or to overwhelm ork mobs.
One idea could be a crossover with the Star Trek universe. The Tyranids could be seen as an extremely hostile alien force that the Federation has to deal with. Their bio - ships and swarming tactics would be a huge contrast to the more technological - based Starfleet.
One idea could be inserting yourself as a new recruit in a Space Marine chapter. You could experience the brutal training and the strict code of honor. Another is to be a tech - priest, exploring the complex machinery and the religious aspects tied to it. Maybe even be a rogue trader, traveling the galaxy in search of profit and adventure.
One idea could be a vampire Space Marine. Maybe a genetic experiment gone wrong in the Imperium that results in a Marine with vampiric traits. He has to struggle with his newfound thirst while still trying to fulfill his duty to the Emperor.
A fanfiction could explore 10k's backstory more in - depth. Maybe he had a family before the apocalypse that he is constantly reminded of during his travels. This could add a whole new emotional layer to his character.
Another interesting concept is to have a Primarch face a completely new enemy. Not the usual chaos forces or xenos, but something from within the Imperium itself that has turned against it. This would create a lot of internal conflict and moral dilemmas for the Primarch to deal with.
One idea could be a crossover with a popular anime like 'Naruto'. Imagine the fighters from 'King of Fighters K' having to battle the ninjas. They could learn new skills from each other. For example, Kyo could learn some shadow clone techniques and use it in his fight style. Another idea is a crossover with a classic video game like 'Super Mario'. The 'King of Fighters K' characters could be in the Mushroom Kingdom and have to deal with Bowser while also facing their own rivals.