One idea could be an Necron Overlord who rebels against the Silent King's plan. He might start to form alliances with other races in secret to gain power and knowledge. This could lead to a whole new set of power dynamics in the Warhammer universe.
Maybe a Necron OC who has regained some of his pre - transformation emotions. He struggles with his newfound feelings while still being part of the Necron race. This internal conflict can create a lot of interesting storylines, like how he deals with his brethren who are still emotionless.
A Necron OC that was once a great scientist before the transformation. Now, he tries to use his knowledge from the past to modify the Necron technology in a way that no one else has thought of. This could involve creating new weapons or even a new form of Necron body that has different abilities.
The appearance and customization of Necron OCs can be really cool in fanfiction. They could have different color schemes or unique body modifications compared to the standard Necrons. For instance, an OC might have a more organic - looking part on their body as a remnant of their pre - Necron existence. Their personality also matters. A Necron OC could be more curious about the galaxy than the typical emotionless Necrons, leading them to explore places and engage in situations that normal Necrons wouldn't.
There might be a fanfiction where a group of Imperial Guardsmen encounter a Necron tomb world awakening. This story could explore the horror and confusion of the guardsmen as they face the ancient and powerful Necrons, and how they try to survive against overwhelming odds. It could also show the Necrons' view of the 'inferior' organic lifeforms.
One popular Necron OC is 'Xarion'. He is often depicted as a Necron lord who has regained some of his former emotions and uses his strategic mind to outwit his enemies. Another is 'Lythia', a female Necron who has a unique bond with a group of Eldar. She tries to find a balance between the Necron's cold logic and the Eldar's more fluid way of thinking.
How about a Necron crossover with the Star Trek universe? The highly advanced and often mysterious Necron technology would be an interesting contrast to the Federation's ideals of exploration and cooperation. The Necrons could be seen as a new and dangerous threat that the Starfleet has to face, perhaps mistaking their motives for aggression when they are really just following their own inscrutable goals.
Begin by deciding on the role your Necron OC will play. Are they a warrior, a scientist, or something else? Once you've got that, consider their personality. Since Necrons are usually cold and calculating, you could give your OC some quirks to make them stand out. For example, if they're a warrior, they might have an obsession with a particular type of combat. Then, outline the story structure, starting with an introduction to your OC and leading into the main action.
One interesting storyline could be the Necron Lord OC's struggle to regain their former glory in a universe that has changed much since their slumber. They might have to face new and powerful enemies while also trying to unite the disparate Necron factions under their rule.
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.
Well, an interesting fanfic idea could involve an OC who is a half - elf. This OC has a natural affinity for beauty and art, which makes them a target for Slaanesh. However, they have a strong bond with their elven heritage that values purity and nature. So, they struggle between the temptations of Slaanesh and their own identity. Also, a story about an OC who is a gladiator in a Slaanesh - influenced arena. They have to fight not only for their life but also against the psychological warfare of Slaanesh's followers who try to convert them.
Well, an interesting plot could involve an OC who is a skilled artisan in the Warhammer world. This OC could be commissioned to create a powerful weapon, but during the process, they discover a dark secret about the materials or the person who commissioned it. Maybe an OC who is a diplomat trying to broker peace between two warring factions, but they face many obstacles and betrayals along the way.
One idea could be a crossover between Warhammer and Lord of the Rings. Imagine the Space Marines facing off against the Orcs of Mordor. Another could be a Warhammer and Star Wars crossover, like the Imperium of Man encountering the Galactic Empire. Or perhaps a crossover with the Witcher, where Witchers team up with Warhammer's warriors to fight common foes.