Definitely Dante from 'Devil May Cry'. Dante is a stylish and powerful demon hunter. His acrobatic combat style and his cocky attitude would contrast well with Death's more stoic and purpose - bound personality. They could end up in a situation where they have to fight a common enemy that threatens both their worlds.
Sora from 'Kingdom Hearts' could be an interesting choice. Sora's pure - hearted and optimistic nature would be a sharp contrast to Death. Maybe Sora ends up in the Darksiders' universe and has to learn from Death about the darker aspects of existence while Death might be intrigued by Sora's unwavering hope and the concept of the heart that Sora represents.
Geralt of Rivia from 'The Witcher' is also a great candidate. Geralt is a monster slayer with a moral code that is often tested. A crossover with Death could involve Geralt being called to the Darksiders' world to hunt a particularly dangerous creature, and he has to work with Death to understand the nature of this new and strange place and the threats within it.
There's a fanfiction where Death crosses over with the 'Warhammer' universe. It's really interesting as it shows how Death's character would interact with the various races and magic systems in Warhammer. His quest for balance in Darksiders 2 takes on a whole new meaning when he has to deal with the chaos and order of the Warhammer world. It also delves into how his scythe might compare to the weapons in Warhammer.
Superman from the DC universe. Superman stands for truth, justice, and the American way, but his methods are often more lenient compared to Akainu. The two could have a philosophical debate about justice while also having to fight a common enemy.
If you're doing a crossover with the Mass Effect series, Shepard could be a great character to include. Shepard's leadership and combat experience against various alien races would be interesting when paired with the Horsemen's fight against the forces of Hell. Also, including some of the alien species from Mass Effect like the Krogan or the Asari could create some really cool interactions.
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.
In a Naruto and Darksiders crossover fanfiction, there are numerous possibilities. For starters, the art of ninjutsu could be seen as a new form of power in the Darksiders' universe. Naruto's ability to befriend his enemies might also play a role. He could try to reason with some of the more morally ambiguous characters in Darksiders. On the other hand, the Darksiders' characters could introduce Naruto to a whole new level of power and danger. Their weapons and abilities are far different from what he's used to in Konoha. This could lead to Naruto growing in new and unexpected ways, perhaps even unlocking new forms of his own power.
Luffy from 'One Piece' would also be an interesting crossover character. Imagine Ichigo being abandoned on an island in the 'One Piece' world. Luffy's carefree yet determined nature could contrast with Ichigo's more brooding personality. They could embark on a journey together, with Ichigo learning about the concept of the 'One Piece' and using his powers in unexpected ways within this new world.
Kakashi from Naruto himself could be part of a 'grey' themed crossover within the Naruto universe. Maybe a situation where Kakashi's past and his more mysterious side are explored further, and Naruto has to work with him in a darker, more complex mission that delves into the grey areas of the ninja world.
Alice from Alice in Wonderland could also be an interesting choice. Her curious nature would lead her to explore the world of Young Dracula. And Young Dracula might be both amused and a bit annoyed by her constant questions and her fearless attitude in the face of his vampiric self.
Piccolo could be an interesting crossover character. He could be from a world where lemons have some sort of magical or spiritual significance. His relationship with Gohan in the original story would add depth to their interactions in this crossover. For example, Piccolo might be initially skeptical of the lemon - related things but eventually see their value when it comes to helping Gohan.
Solid Snake from the Metal Gear series would be a great choice. His stealth skills and military expertise would contrast with Travis Touchdown's more brash and direct fighting style. They could be forced to work together on a mission, and learn from each other's ways of combat.