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.
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.
One interesting idea could be a crossover with "Moana". Elsa's ice powers and Moana's connection to the ocean could create a really cool dynamic. Maybe Elsa accidentally freezes part of Moana's ocean and they have to work together to fix it. Another could be with "Tangled". Rapunzel's long hair and her adventurous spirit combined with Elsa and Anna's journey in Frozen 2 would be exciting. Rapunzel could use her hair to help them climb mountains or cross chasms. A third idea is a crossover with "Beauty and the Beast". Belle's love for books and knowledge could help Elsa understand more about her powers and the history of magic in the Frozen world.
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 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.
There might be a necron fanfic where the Necrons form an unexpected alliance with another race. For example, they team up with the Tau in order to fight a common enemy like the Tyranids. This alliance brings about a lot of cultural clashes and strategic challenges for both sides.
Trazyn the Infinite from the Necrons would be a great character to feature. His obsession with collecting and preserving all kinds of curiosities could lead to some really interesting crossovers. For example, if crossed with a character from Doctor Who, he might be interested in collecting some of the Doctor's time - traveling gadgets.
A Rambo crossover fanfiction could involve him crossing over into the Star Wars universe. Rambo's guerilla warfare tactics could be interesting in a galaxy far, far away. Maybe he helps the Rebel Alliance in a ground - based battle against the Empire. Or a crossover with the Predator series. Rambo would be a worthy opponent for the Predator, and the fight between them in the jungle could be epic. Also, a Rambo and John Wick crossover would be intense, two highly skilled fighters on the same side or perhaps initially at odds.
One idea could be a crossover with a modern military setting. The Decepticons could be trying to steal advanced military technology. Another is a crossover with a post - apocalyptic world where they are competing with other factions for resources. A third could be a crossover with a magical realm, with the Decepticons trying to harness the magic for their own evil purposes.
One idea could be a crossover with Naruto. Kakuzu could be sent to the Naruto universe and form an unlikely alliance with a shinobi village to fight a common enemy, like the Akatsuki from his own world turned rogue in the new universe. This would create a lot of interesting character dynamics and battles.
Another option is a crossover with 'The Little Mermaid'. The characters from the descendants could meet Ariel and her friends underwater. Evie might be interested in the mermaid fashion and could design new outfits inspired by the underwater world.