It can start with mutual distrust. Doomguy is used to killing demons, and the female demon is afraid of him. But when they are forced into a situation where they have to rely on each other to survive, like being trapped in a small area surrounded by more powerful demons, they start to see each other in a new light. They begin to communicate, first out of necessity, and then gradually become more open with each other.
The Doomguy was once a marine. He was sent to Mars to investigate some strange happenings on a Union Aerospace Corporation (UAC) facility. The UAC had been messing around with teleportation technology and had accidentally opened portals to Hell. Doomguy found himself in the middle of a demonic invasion. His initial experiences on Mars were filled with fighting off hordes of demons, and this was just the start of his journey to become the ultimate demon - slayer.
First, get to know the characters well. For RWBY, study their personalities, abilities, and backstories. For Doomguy, understand his motives and his badass nature. Then, think of an interesting scenario where they could meet, like a rift opening between their worlds.
A lot of the time, Doomguy's indomitable will and his combat experience in the face of demons can be paired with Naruto's ninja skills. Picture Doomguy charging into battle, and Naruto using his wind - style jutsu to enhance Doomguy's attacks, like making his melee strikes even more powerful with gusts of wind.
Doomguy is a very silent and rage - filled character. He just wants to kill demons. Goblin Slayer is more focused on goblins specifically. Their personalities might clash at first because Doomguy might see goblins as too easy of a target compared to demons. But they could blend as they both have a single - minded determination in combat.
Doomguy is known for his brute strength and his arsenal of powerful guns. In comparison, RWBY characters have more diverse abilities based on Dust and their Semblances. For example, Ruby has her speed - enhancing Semblance. Doomguy's guns would be a huge advantage in terms of raw firepower, but the RWBY characters' abilities offer more flexibility in combat.
There's a fanfic called 'Doomguy in the Hidden Leaf'. In this one, Doomguy gets transported to the Naruto universe. It's interesting to see how he interacts with the different ninja clans, especially when his no - nonsense attitude clashes with the more traditional ways of the Leaf Village. It's a short but engaging read.
The key elements include the Union Aerospace Corporation (UAC). Their unethical and dangerous experiments with teleportation on Mars were crucial. Doomguy being a marine is also important as it provided him with the discipline and some level of combat ability. The sudden demonic invasion that he had to face on Mars was the turning point. It forced him to adapt and start his journey as the Doomguy. Without these elements, his origin story would be completely different.
Well, in a 'naruto is doomguy' fanfiction, Naruto would find himself in a completely different and much darker world than his own. He'd have to quickly learn how to handle Doomguy's arsenal of weapons. His natural athleticism and combat skills as a ninja would be put to the test in a new way against the demonic hordes. The story could also involve Naruto trying to find a way back to his own world while dealing with the constant threat of the demons. It could be a story of adaptation, survival, and heroism as Naruto tries to use his unique skills to fight in a world that's not his own.