Well, it could be about the RWBY characters finding themselves in the Doom universe. Maybe Ruby and her team are trying to fight off the demons in Doom while also using their own unique abilities. They might team up with the Doom Slayer at some point to take on a big, bad enemy that threatens both universes.
In a RWBY Doom crossover fanfiction, the plot could involve the Grimm from RWBY being somehow connected to the demons in Doom. The RWBY characters could be shocked by the extreme violence and gore of the Doom world at first. But then they learn to adapt and use their skills like semblances in new ways. For example, Weiss could use her glyphs to freeze demons in place while the Doom Slayer slices them up. And they all work together to find a way to close the portals that are bringing the two menaces together.
The story could start with a strange rift opening up between the RWBY and Doom universes. Yang, being the brash one, might be the first to jump into the fray in the Doom world. She discovers that her punches are not as effective as she thought against the powerful demons. Meanwhile, Blake uses her stealth to scout out the situation. They soon realize that they need to combine their powers with the Doom Slayer's arsenal. The Doom Slayer, on the other hand, is initially skeptical of these strange new allies but comes to respect their abilities as they fight side by side to save both their worlds from destruction.
The plot could involve the RWBY team being confused at first in the Doom world. They then start to adapt and learn about the weapons and power - ups in Doom. For example, Weiss could be intrigued by the high - tech weaponry and try to figure out how to incorporate it with her Glyphs. Blake might use her stealth to navigate the dangerous levels, and they all work together to survive the Doom chaos.
A possible plot is that a portal opens between the Doom universe and RWBY's. Some of the Doom demons start spilling into Remnant, the world of RWBY. Ruby and her team initially mistake them for a new, extremely powerful type of Grimm. As they try to fight them, they realize they need different strategies. Meanwhile, the Doom Slayer follows the demons through the portal to stop them. Eventually, the two groups meet and learn from each other's combat styles to defeat the invading demons.
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 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.
Maybe the Hazbin Hotel is in the middle of a crisis where the souls of the damned are being consumed in a strange and new way. Charlie and her crew suspect it has something to do with a portal that has opened up to a dimension filled with the kind of demons the Doom Slayer is used to fighting. They seek out the Doom Slayer, who has to navigate the politics and oddities of Hazbin Hotel's Hell while trying to close the portal and stop the consumption of souls.
One possible plot could be that the RWBY characters find themselves in the Fallout wasteland. Ruby could use her scythe skills to fight off the mutated creatures in the wasteland. Weiss might struggle with the harsh and dirty environment compared to her usual posh settings but use her glyphs in unique ways against raiders. Blake could use her stealth to navigate dangerous areas, and Yang's strength would be useful in hand - to - hand combat with the tough Fallout foes.
One possible interesting plot could be that the Doom Slayer is somehow transported to the world of High School DxD. He could end up at the school as a new student, but his combat skills and his no - nonsense attitude would quickly set him apart. Maybe he gets involved in the battles between the different factions in the DxD world because they mistake his presence for a threat or an opportunity. He might use his arsenal of weapons from Doom to take on the devils, fallen angels and other supernatural beings in the DxD universe in his own unique way.
One possible plot could be that Doflamingo somehow ends up in the RWBY world. He might initially try to take over, using his strings to manipulate the Grimm. But the RWBY team discovers his true nature and battles him. Ruby could use her speed to avoid his string attacks while Weiss uses her glyphs to counter his powers.
Another plot could be that an evil force in the RWBY universe discovers the existence of the Omnitrix from another dimension. They manage to create a device to pull Ben and the Omnitrix into their world. Ben has to adapt quickly to this new world while Team RWBY, who are initially suspicious of him, realizes that he could be an ally against this new threat. They join forces, with Ben using his alien forms in creative ways to combat the Grimm and the evil force's minions.