Definitely Geralt from The Witcher. His knowledge of monsters and magic could complement the Doom Slayer's brute force. They could work together to hunt down some powerful demons that are causing chaos in both their worlds.
Definitely Doomguy as one of the main characters. He is the iconic figure of the Doom series and his presence is essential. Another could be Lara Croft from the Tomb Raider series. Her skills in exploration and combat could complement Doomguy's abilities. They could be on a quest to stop an ancient evil that has ties to both their worlds, like a demonic force that is also seeking some powerful artifacts that Lara has been studying.
Well, one possible plot could be that Doom Slayer somehow gets transported to the Fairy Tail universe. He might initially be seen as a threat due to his intimidating appearance and his violent ways of dealing with enemies. But then, as the Fairy Tail members get to know him, they realize he's fighting against a greater evil. They could team up to take on a powerful dark wizard who has made a pact with demons similar to those Doom Slayer fights. The contrast between the magic - based world of Fairy Tail and the high - tech, gore - filled world of Doom would make for an interesting story.
Good characterization is key. The Doom Slayer should stay true to his in - game persona - a silent, unstoppable force. For example, if he crosses over with a more talkative character from another franchise, the contrast should be well - written.
Well, one idea for the plot of such a crossover fanfiction could be that the Doom Slayer gets transported to the world of Wolfenstein. He then has to figure out how to deal with the different type of enemies there, the Nazis. Meanwhile, the Wolfenstein hero could be intrigued by the Doom Slayer's advanced weaponry and otherworldly strength. They might start off as cautious allies, but as they face more and more challenges together, they form a strong bond and a united front against the combined evils of demons and Nazis.
One possible plot could be that the Doom Slayer is somehow transported to the world of Goblin Slayer. He then teams up with Goblin Slayer to take on an especially powerful horde of goblins that have been corrupted by demonic forces from Doom. Their combined skills in combat, the Doom Slayer's superhuman strength and arsenal, and Goblin Slayer's knowledge of goblin tactics would make for an epic battle.
One idea could be a crossover with the Halo universe. The Doom Slayer could team up with Master Chief to fight against a common alien threat that has invaded both their worlds. They could share their combat skills and weaponry knowledge.
One great Doom Slayer crossover fanfiction is the one where he crosses over with the Halo universe. It's exciting to see the Doom Slayer's brute force combined with the high - tech weaponry of Halo. The way the two different art styles and storylines blend is really engaging.
One interesting aspect could be the contrast in their fighting styles. The Goblin Slayer is more methodical and strategic when dealing with goblins, using his knowledge of their habits. The Doom Slayer, on the other hand, is a force of pure destruction. In a crossover, perhaps they could learn from each other. The Goblin Slayer could pick up some of the Doom Slayer's more aggressive tactics, while the Doom Slayer might gain some strategic thinking from the Goblin Slayer.
In a crossover with Harry Potter, Harry Potter himself could be a great addition to a Demon Slayer crossover. His magical skills and ability to face dark forces could be useful against demons. Also, Hermione with her vast knowledge could help find ways to defeat demons more effectively. In a crossover with Dragon Ball Z, Goku would be amazing. His super strength and ability to fly would give the Demon Slayers a huge advantage in battles. And Vegeta's pride and combat skills would also be a great asset.
Sora from Kingdom Hearts could be an interesting choice. Sora's positive and heroic nature would contrast with Ragna's more brooding personality. They could team up to fight against the Heartless that somehow invaded Ragna's world or vice versa.