A cool doom fanfic idea might be to write about a group of scientists who were studying the portals that the demons come through in the Doom world. They accidentally open a new type of portal that brings in a whole new breed of demons, and they have to team up with the Doom Slayer to figure out how to close it. Or, you could write a fanfic where the Doom Slayer travels to an alternate dimension that is even more hellish than his usual battlegrounds. There, he discovers new powers and weapons to use against the demons. Also, a fanfic could focus on a character who is cursed to be part - demon and part - human, and their struggle to fit in either world while also trying to stop the demon invasion.
One interesting idea could be a doom fanfic where the protagonist is a new recruit in the Doom Slayer's army. They have to learn the ropes and face off against demons in a training scenario that goes horribly wrong, leading to an unexpected adventure. Another could be a fanfic set in a post - Doom world where the remaining humans are trying to rebuild and there are still hidden pockets of demonic activity that they have to deal with. And a third idea could be a fanfic that explores the backstory of one of the demons, perhaps giving it a more sympathetic angle before it became a monster in the Doom universe.
Some good doom fanfic ideas include a story where the Doom Slayer is somehow stripped of his powers and has to rely on his wits and the help of a ragtag group of survivors to get them back. This could involve a lot of stealth and strategy as he can't just go in guns blazing. Another idea is to create a fanfic that delves into the relationship between the Doom Slayer and the UAC (Union Aerospace Corporation). Maybe there are some secrets within the UAC that the Doom Slayer uncovers that are related to the origin of the demons. Also, you could write a fanfic about a female character who takes on the mantle of the Doom Slayer in a parallel universe where the original Doom Slayer doesn't exist, and she has to prove herself against the hordes of demons.
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.
Perhaps a crossover with a fantasy world. Doomguy enters a realm filled with magic, elves, and dragons. He might initially be seen as an outlander, but as he battles the demons that have also invaded this fantasy world, the inhabitants start to see him as a valuable ally. The clash between Doomguy's advanced weaponry and the magic of the fantasy world would be a fascinating aspect to write about.
One idea could be a crossover with the Halo universe. Imagine the Doom Slayer facing off against the Covenant. Another could be a crossover with the Warhammer 40K universe, where the Doom Slayer battles Chaos forces in a hellish landscape that combines both universes. Or a crossover with the Mass Effect universe, where the Doom Slayer's brutal combat style clashes with the more strategic and technological approach of the Mass Effect races.
One idea could be creating an OC who is a survivor from a previously unmentioned colony on Mars. This OC could have unique skills like advanced engineering, which they use to build makeshift defenses against the demons. Another idea is an OC that has a special connection to an ancient artifact on Doom's Earth, and this connection gives them the ability to control certain types of demons or at least influence their behavior. A third idea is an OC who is a part of a secret resistance group within Hell itself, trying to undermine the demon hierarchy from the inside.
Another possibility is a crossover with the Mass Effect universe. The Doom Slayer arriving in the Milky Way and getting involved in the fight against the Reapers. His brute force approach might shock the more tactical Alliance forces, but it could also be the key to victory in some battles.
Some doom crossover fanfic ideas include a crossover with the Dark Souls universe. The Doom Slayer could be in the gloomy and challenging world of Dark Souls, facing off against tough bosses and hordes of undead. Another idea is a crossover with the Devil May Cry series. The Doom character might team up with Dante or Vergil to take on demonic threats. And a crossover with the Mass Effect universe could be about the Doom hero getting involved in the fight against the Reapers and other intergalactic menaces.
One idea could be the Doom Slayer arriving in the RWBY world. He could be on a mission to stop some sort of demonic invasion that has seeped into Remnant. His powerful arsenal and unwavering determination would be a huge contrast to the Huntsmen and Huntresses of RWBY.
A really interesting fanfiction plot could be about a group of scientists who were working on a project to reverse - engineer the demon technology. They think they can use it to create a weapon against the demons. However, things go wrong and they accidentally open a new portal to a different demon dimension, which brings in even more powerful and unknown demons. The story could then focus on how they deal with this new threat and try to close the portal.
One idea could be a scenario where the Doom Slayer somehow ends up in the Halo universe. He might team up with the Master Chief to fight a common enemy that combines the demons from Doom and the Covenant or Flood from Halo. For example, a new threat that has the ability to corrupt both the technology in the Halo universe and summon Doom demons.
In a Doom and Wolfenstein crossover fanfiction, it could be that there is a portal that opens between the two worlds. The demons start pouring into the Wolfenstein world, and BJ has to reach out to the Doom Slayer for help. The Doom Slayer brings his arsenal of powerful weapons, and together they train the resistance fighters in Wolfenstein to use these weapons. They also discover that some of the Nazi technology can be modified to be more effective against the demons, creating a unique blend of combat strategies from both universes.