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.
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 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.
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 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.
Well, a cool plot for a Witcher RWBY crossover fanfiction might involve the Witcher's magic and potions being introduced to the RWBY universe. For example, a potion that gives the RWBY characters temporary enhanced senses like a Witcher. Or perhaps a situation where a powerful Witcher villain comes to the RWBY world and the teams have to combine their skills to defeat him. The different fighting styles of the Witcher and RWBY characters would make for some really exciting battle scenes.
Well, another plot could involve Sans and Papyrus coming to the RWBY world. Papyrus might be fascinated by the Grimm and try to befriend them, while Sans could use his powers to help the RWBY team in their battles against Salem. He could also share his knowledge of timelines with Ozpin.
One idea could be the Overwatch heroes being transported to the RWBY world. They could team up with Team RWBY to fight a new common enemy, like a powerful Grimm that is immune to normal attacks but vulnerable to Overwatch tech. Another plot could involve a character exchange, where a RWBY character like Ruby goes to the Overwatch world and learns about their technology and fighting styles, and then brings back new knowledge to her own world.
One idea could be Cinder being transported to an alternate universe within the RWBY world where the roles are reversed. For example, she could find herself in a world where she is a huntress in training, facing the same challenges as the main characters but from a different perspective. Another plot could involve a team - up between Cinder and a group of RWBY characters against a common, greater enemy that threatens both their goals and the world. Maybe a new, powerful Grimm that has abilities they've never seen before.
One idea could be that the Dragonborn is somehow transported to the RWBY world. They could use their shouts to combat the Grimm. For example, 'Fus Ro Dah' could send Grimm flying back, and the local Huntsmen and Huntresses are both amazed and a bit wary of this new power.