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 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.
Madara would be a very powerful and unique addition. His combat skills are top - notch. In the Fairy Tail universe, he could be a rival to some of the strongest mages at first. His Sharingan would give him an edge in battles. For instance, he could use it to predict the movements of opponents like Erza or Gildarts. However, he would also have to adapt to the magic - based combat system of Fairy Tail.
One great 'Naruto Madara Uchiha Crossover Fanfiction' is 'The Uchiha's New Path' on FanFiction.net. It combines Madara's power with an alternate universe concept where he teams up with characters from other anime. It's really interesting to see how his skills and personality interact with new characters.
One great 'madara uchiha crossover fanfiction' is the one where he crosses over into the Naruto and Bleach universe. It's really interesting to see how his powerful Uchiha abilities interact with the different powers in the Bleach world like Soul Reaper and Quincy abilities.
Sasuke would be a very powerful addition to the RWBY world's power structure. His Sharingan gives him enhanced perception, which would be extremely useful against the Grimm. He could easily analyze their attack patterns and weaknesses. Also, his ninjutsu skills like the Chidori and Fire Style: Great Fireball Jutsu are very powerful. In terms of the Huntsmen and Huntresses ranking, he would probably be considered an S - class level threat or fighter due to his abilities.
Often, Naruto might be paired with Madara in some sense. Their opposing views on power and peace could create interesting interactions in the fanfiction.
Ruby would probably be excited by the new and different kinds of challenges in the Fallout world. She'd quickly learn to use the weapons she finds there while still relying on her Crescent Rose. Weiss would have a harder time adapting to the dirt and grime, but her determination would make her find ways to use her magic in this new environment. Blake would likely be more at ease in the shadows of the ruined buildings, using her stealth to her advantage.
In a crossover with the Avengers, Thor would be an interesting character to pair with Madara. Thor has his godly powers and Mjolnir, while Madara has his Sharingan and immense combat skills. The clash between Thor's lightning and Madara's fire - based jutsu would be epic. Another good pairing could be with Batman. Batman's strategic mind against Madara's cunning and power would make for a great dynamic. Batman would have to rely on his gadgets and intelligence to outwit Madara.
Ruby would probably be very curious at first. Her adaptable nature would help her start learning about the apes' ways quickly. Weiss might have some trouble due to her more refined upbringing, but she could use her intelligence to find common ground. Blake, with her experience of being an outcast, could potentially understand the apes' struggle for their own place in the world better. Yang would likely be ready to fight if the situation called for it, but also try to find peaceful solutions.
One common theme is power dynamics. Madara is a very powerful character, so in the harem, there are often issues of how his power affects his relationships with the other members. Another theme is cultural clash. Since it's a crossover, characters from different backgrounds come together, which can lead to interesting cultural exchanges within the harem.