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.
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.
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.
Dante from Devil May Cry would be an excellent character to pair with Doomguy. Dante is a stylish demon hunter, and when combined with Doomguy's brute force approach, it would create some really cool action scenes. Also, Samus Aran from Metroid could be a good match. Samus' exploration skills and Doomguy's combat abilities could lead to an interesting story, especially if they are in a shared alien - infested environment.
A cool Doomguy crossover fanfiction could involve him crossing into the Dark Souls world. He'd be this indomitable force in a land of dark and difficult challenges. He could also cross over with the God of War universe. Doomguy and Kratos would make an interesting pair, both being extremely powerful warriors with a lot of anger. And how about a crossover with the Borderlands? Doomguy could run amok in the chaotic world of Pandora, fighting both demons and bandits.
There are definitely some out there. Check out various online writing communities. Some writers might explore how Doomguy's combat skills would fare against the threats in the Clone Wars. Maybe he'd team up with Jedi or face off against Sith, leading to some really exciting and unexpected storylines in these fanfictions.
One common theme is the clash of different power systems. Doomguy has his own brute - force combat style and powerful weapons, while the Clone Wars has the Force and lightsabers. Another theme could be the idea of a lone warrior (Doomguy) in a large - scale war (Clone Wars).