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.
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.
One idea could be that the Doom Slayer is transported into the Warhammer universe. He might start by facing off against hordes of Orks in a brutal melee, using his signature weapons. His superhuman strength and unwavering determination would make him a force to be reckoned with among the various factions.
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.
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.
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.
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.
One idea could be the characters from RWBY ending up in the world of Overlord. They might initially be confused and have to adapt to the new, more dangerous environment. Maybe Ruby could mistake some of the Overlord's minions for Grimm and attack, causing a stir among the various factions in the Overlord world.
One idea could be that the characters from RWBY are transported into the Destiny universe and have to learn to use the unique weapons and powers there. For example, Ruby could be fascinated by the Exotic weapons in Destiny and try to incorporate their technology into her Crescent Rose. Another plot could involve the Guardians from Destiny being sent to the RWBY world to help fight against the Grimm. They would have to adapt to the new environment and team up with the RWBY characters. The third idea is that a powerful enemy from the Destiny universe somehow crosses over to RWBY's world and the combined forces of both universes have to stop it.
Maybe the Persona users' powers start to malfunction in the RWBY world due to the different nature of the place. So they have to figure out how to control their powers again while also helping the RWBY characters in their battles. The RWBY characters could be intrigued by the Persona powers and try to learn from them.