One idea could be creating an OC who is a hybrid of different races in the Darksiders universe. For example, a half - angel, half - demon character. This would bring a unique perspective as they struggle with their dual nature, being neither fully accepted by the angels nor the demons.
Another interesting approach is to make an OC that has a special power related to the balance of the universe, like the ability to detect when the forces of good and evil are tipping too far in one direction. They could then embark on a journey to restore the balance, interacting with the main Darksiders characters along the way.
Another plot could involve a crossover within the Darksiders universe. Maybe the Horsemen have to team up with a previously unknown group of characters to face a new, greater threat. This could introduce new dynamics and relationships.
One idea could be a crossover where the characters from Naruto end up in the Darksiders universe and have to fight the demons there using their ninja skills. For example, Naruto could use his shadow clones to confuse the hordes of demons while Sasuke uses his Sharingan to predict their moves.
Perhaps the Avengers mistake one of the Horsemen for a new super - villain. As they engage in battle, they soon realize that the Horsemen is actually after a powerful artifact that a real villain in the Avengers' world has obtained. So they form an uneasy alliance to retrieve the artifact.
Another interesting concept for a Darksiders War crossover fanfiction could be a crossover with the Diablo series. Imagine the dark and gritty world of Diablo mixing with the post - apocalyptic world of Darksiders. The heroes from Darksiders could be lured into the world of Sanctuary in Diablo, where they have to deal with the hordes of demons while also trying to find a way back home.
One idea could be that the Horsemen from DarkSiders end up in the world of RWBY. Maybe War arrives first and is mistaken for a powerful Grimm. He then has to figure out how to interact with the Huntsmen and Huntresses while also trying to find a way back to his own realm. Another idea is that the characters from RWBY somehow stumble into the post - apocalyptic world of DarkSiders and have to adapt to the harsher and more dangerous environment.
Another possibility is a crossover with 'God of War'. Death and Kratos could meet. Given their both powerful and somewhat vengeful natures, they could either become bitter enemies or reluctant allies. The story could explore their different views on death, with Death being an embodiment of it and Kratos having a very personal and violent relationship with death in his own journey.
One plot idea could be Uriel embarking on a quest to find a lost relic in the Darksiders universe. Along the way, she encounters various challenges and forms an unexpected alliance with a character who has a mysterious past. This could lead to a lot of character development and exploration of the Darksiders world.
One idea could be exploring the backstory of War before the events of the first game. Maybe show his training and early battles that shaped him into the Horseman we know.
One interesting fanfiction could be about the origin of the Horsemen in a more detailed and personal way. It might explore their relationships before they became the apocalyptic riders.
Perhaps there is a dark force corrupting the angels, and Uriel is suspected of being influenced. War is sent to investigate. As he gets closer to Uriel, he realizes she is innocent but they must work together to find the true source of the corruption and stop it before it spreads throughout the entire angelic realm. This would involve a lot of action and tense moments as they fight against both the corrupted angels and the real threat behind it.