One plot idea could be that Black War Greymon from the Digimon universe crosses over into a different fantasy world. He might have to team up with the local heroes to fight against a common enemy that threatens both his world and this new one. For example, an ancient evil force that has the power to destroy all digital and non - digital life forms.
Perhaps in the fanfic, Black War Greymon could cross over into a post - apocalyptic world. Here, he meets a group of survivors who initially fear him but then realize his strength can be used to fight against the mutated creatures that roam the land. The story could focus on his journey to regain his purpose in this new world and how he forms deep bonds with the humans he protects.
Another plot could involve Black War Greymon getting stranded in a modern - day city in a crossover fanfic. He has to figure out how to adapt to this strange new environment while also being hunted by a secret government agency that wants to study his powers. At the same time, he discovers that there are other hidden Digimon in the city who he can form an alliance with.
First, do your research on Black War Greymon. Know his powers, his backstory, and his personality traits. This will help you write him accurately in the crossover. For example, if he's known for his brute strength and solitary nature, you can use that in the new setting. Next, create a unique and engaging crossover world. It could be a mash - up of two different anime universes or a completely new world that Black War Greymon enters. Then, develop a strong plot with conflict. Maybe he has to fight against the rules of this new world or face a powerful antagonist.
Power struggles are also quite common. Given his powerful nature, fanfics might involve him in battles or power - related conflicts within the Digimon universe. For example, he could be fighting against other powerful Digimon to prove his worth or for some greater purpose.
You can try searching on fanfiction websites like FanFiction.net or Archive of Our Own. There are often many Digimon - related fanfics there, and you should be able to find some specifically about Black War Greymon.
One idea could be a crossover with the Harry Potter universe. The Avengers could travel to Hogwarts in search of a magical relic that could help them in the fight against Thanos. Maybe Doctor Strange uses his magic to open a portal to Hogwarts and they team up with the wizards to defend the relic from Thanos' minions.
A crossover with a magical girl anime might also be cool. Black Rock Shooter could find herself in a world full of magic and have to adapt her fighting skills. Maybe she has to help the magical girls defeat a dark force that is immune to their usual magic, using her own brute strength and determination.
One idea could be a crossover with the Halo universe. Imagine the COG soldiers teaming up with Spartans to fight a common enemy that threatens both their worlds. They could share their combat strategies and technologies, like the Lancer's chainsaw bayonet with the Halo's energy weapons.
A God of War and Assassin's Creed crossover fanfic could be really cool. Picture Kratos in the world of Assassin's Creed, with his brute strength and combat skills clashing or collaborating with the assassins' stealth and parkour abilities. He might be on a quest that somehow intersects with the Assassin's Creed's fight against the Templars.
One idea could be Goku Black crossing over into a modern-day Earth where he has to deal with advanced technology that he doesn't understand at first. Another could be a crossover with a different anime universe, like Naruto, where Goku Black's power clashes with the ninja abilities. Maybe he teams up with Naruto against a common enemy.
A cool plot for a 'Black Butler' crossover fanfiction might be a crossover with 'Kuroshitsuji' and 'Death Note'. Light Yagami could somehow cross paths with Ciel and Sebastian. Light might see Ciel as a means to an end in his grand scheme of creating a new world order, while Sebastian would be intrigued by Light's Death Note and the power it holds. It would be a battle of wits between them, with Ciel caught in the middle, trying to figure out how to use this situation to his advantage.
Another option is a crossover with the Pokémon world. The Phantom Thieves could travel to a region in the Pokémon world and discover that someone is using Persona powers to make Pokémon act out of control. They would need to catch Pokémon themselves and use their combined powers of Personas and Pokémon to stop the bad guy. It would be interesting to see how they interact with Pokémon trainers and the whole Pokémon ecosystem.