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.
Another possibility is that a portal opens between the two universes. RWBY characters end up in the Doom universe first. There, they have to adapt to the much more hellish and violent environment. Weiss could use her Glyphs in creative ways to fight the demons, while Yang would rely on her brute strength. When they bring the Doom Slayer back to Remnant, the power balance in the fight against the Grimm is completely changed.
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 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.
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 interesting idea could be a crossover of different timelines within RWBY. For example, characters from the past meet the current ones and have to deal with the differences in their worlds. Another could be a crossover of different teams in unexpected ways, like Team RWBY having to team up with a rival team to face a common enemy that is more powerful than any they've faced before. And sometimes, there are crossovers that involve alternate versions of the characters, where their personalities or abilities are slightly different, which can lead to very unique and engaging storylines.
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 that Percy and his friends from Camp Half - Blood are somehow transported into the Doom universe. Maybe they have to fight the demons there using their demigod powers and the weapons they find in the Doom world. For example, Percy could use his water powers to fight the fire - based demons.
One interesting idea could be the unique character interactions. For example, how the RWBY characters might interact with the denizens of 'heaven' in this fanfiction. Maybe they learn new skills or gain different perspectives on their own world.
One idea could be that Superman arrives in the RWBY world and has to learn about Dust and its powers. He could team up with Team RWBY to fight a common enemy, like a powerful Grimm that is immune to normal attacks but not to Superman's strength. Another idea is that Ruby discovers she has some Kryptonian-like abilities and Superman helps her understand and control them.