I think it would be cool to pair the Doom Slayer with Skarbrand. They are both extremely powerful and have a thirst for destruction. It would be an epic battle of two titans, and depending on how the fanfiction goes, they could end up having an uneasy alliance or just keep trying to kill each other.
Definitely the Emperor of Mankind. The contrast between the all - seeing, all - powerful (sort of) Emperor and the single - minded, brutally efficient Doom Slayer would be fascinating. They could have a complex relationship, with the Emperor trying to direct the Slayer's rage for the 'greater good' of the Warhammer universe.
The Doom Slayer and a Space Marine would be really interesting. They both are combat - focused and have a strong sense of duty. They could start off as rivals, each thinking their way of fighting is superior, but then come to respect each other after facing a common enemy like the forces of Chaos.
The Doom Slayer and Charlie would be an interesting pair. Charlie is all about redemption and trying to make Hell a better place, while the Doom Slayer is focused on pure destruction of demons. Their meeting could lead to some great character development. Charlie might try to convince the Doom Slayer that not all demons are irredeemable, while the Doom Slayer could show Charlie the true horror that some demons are capable of.
However, Denji as Chainsaw Man also has his own set of powers. He can transform into a powerful chainsaw - like being and has shown great resilience. But compared to the Doom Slayer, he lacks the same level of experience in dealing with the kinds of threats that the Doom Slayer has faced on a regular basis. So, overall, the Doom Slayer would probably be more powerful in a direct comparison.
I think the Doom Slayer would be more dominant. His suit gives him enhanced strength, speed, and durability. He has weapons that can level entire areas. While the Goblin Slayer is highly skilled at what he does, the Doom Slayer has fought on a much larger scale against far more dangerous foes. His presence alone would probably intimidate most of the goblins, and his ability to take on hordes of enemies without flinching makes him a very dominant force in any encounter.
It's likely the Doom Slayer. He has proven to be an unstoppable force against hordes of demons. His strength, speed, and arsenal of weapons make him a formidable opponent. In a collision with Warhammer 40K, he could take on many of the threats there as well.
Another interesting interaction could be between Vaggie and the Doomguy. Vaggie is protective of Charlie and the hotel's mission. When she meets Doomguy, she might be suspicious of his motives. She could be constantly on guard around him, and he might be a bit annoyed by her over - protectiveness. Their relationship could start off as very tense but could develop in interesting ways as they get to know each other better.
I think Yang would be the most interesting. Her hot - headed and powerful personality would fit well in the Doom setting. She'd be charging into battles with the demons, using her Ember Celica to blast them away. And her ability to get stronger when she's angry could be a great asset in the high - intensity combat of Doom.
Blake could be a very interesting character in a RWBY Doom crossover fanfiction. Her ability to create illusions and her cat - like reflexes could give her an edge in the Doom universe. Also, her complex backstory and her more reserved nature compared to some of the other RWBY characters would make for an interesting dynamic when interacting with the Doom Slayer and the other denizens of the Doom world. She might be able to use her stealth to sneak up on demons or create illusions to distract them while the others attack from different angles.
Another interesting pairing would be Doom Slayer and Erza. Erza is a strong and disciplined mage. She would be intrigued by Doom Slayer's unwavering determination. They could bond over their shared sense of justice. Erza might even try to learn from Doom Slayer's single - minded focus on destroying evil. And Doom Slayer could be impressed by Erza's ability to handle various types of magic and her leadership within the Fairy Tail guild.
The Doom Slayer and Ruby. Ruby is young and full of hope, always looking for adventure. The Doom Slayer is a stoic, battle - hardened warrior. Their interaction would be really interesting. Ruby could be in awe of his strength and weapons, and the Doom Slayer might see a bit of his own unwavering will in Ruby.