One idea could be Madara being transported to the RWBY world and initially being seen as a great threat. But then he starts to understand the unique nature of Dust and tries to use it in his own jutsu. Another plot could involve him training some of the RWBY characters, like Ruby, in his powerful Uchiha techniques, which would give them an edge against the Grimm.
One idea could be a crossover with the Harry Potter universe. Madara Uchiha could be a powerful dark wizard from a far - off land who arrives at Hogwarts. He might be intrigued by the magic there and try to either conquer or learn from it. Another idea is a crossover with the DC universe. He could be pitted against Superman, and his Sharingan powers would make for an interesting battle against the Man of Steel. He might also form an alliance with Lex Luthor, thinking they have similar goals of power and domination.
Madara would have to adapt to the different threats in RWBY, mainly the Grimm. In his world, he was used to fighting other shinobi and clans. The Grimm are a whole new type of enemy. However, his Sharingan would give him an advantage in detecting their movements. He might also try to find a way to use the Grimm's negative energy, similar to how he used negative emotions in his own world to fuel his power.
One idea could be an OC who is a time - traveler and meets Uchiha Madara in his prime. The OC could have knowledge of future events and try to change the course of history, leading to many power struggles and moral dilemmas.
One idea could be a time - travel plot where Sakura is sent back in time and meets Madara. She tries to change his path of destruction. Another could be a scenario where Madara is resurrected in Sakura's time and they have to team up against a new common enemy. Maybe a plot where Madara, in his search for power, discovers Sakura's unique medical and combat abilities and tries to use her for his own gain.
Well, a plot could involve Sakura and Madara being forced to team up against a new, powerful enemy that threatens the entire ninja world. Sakura initially distrusts Madara, but as they fight side by side, she begins to see a different side of him. Maybe there's a plot where Madara is cursed, and Sakura is the only one who can break the curse because of her unique chakra. As she tries to break the curse, they get to know each other better. Or perhaps in a parallel universe, Sakura and Madara are lovers from the start and they have to fight against the forces that want to tear them apart.
One possible plot could be that Madara Uchiha somehow gets transported to the world of Fairy Tail. He might initially be seen as a threat due to his powerful and somewhat menacing appearance. But as the story progresses, he could form an unlikely alliance with some of the Fairy Tail members. For example, he might team up with Natsu to fight a common enemy. Madara's Sharingan abilities could be a great asset in battles against dark guilds or other powerful foes in the Fairy Tail universe.
One idea could be a love triangle where the OC is caught between Madara's grand ambitions and Izuna's more personal loyalty. The OC might be a healer who tries to keep the peace between the two brothers while falling in love with both in different ways. For example, Madara's strength and leadership might attract the OC on one level, while Izuna's kind heart and protectiveness draw her in another way.
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, 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.