One idea could be that Saitama from OPM ends up in the RWBY world and participates in a Grimm - slaying tournament. His overwhelming strength would shock everyone, and he might just be in it for the free food at the event.
For example, if Saitama from OPM and Yang from RWBY were to fight together. Yang's semblance of getting stronger with each hit she takes could be interesting when combined with Saitama's already extreme strength. Saitama could observe Yang's fighting style and be impressed by her tenacity.
First, choose the other universe you want to cross over with OPM. Then, study the characters, settings, and rules of both worlds. For example, if it's an OPM and Harry Potter crossover, know the magic system in Harry Potter and the power levels in OPM. Next, create a plot that makes sense for the characters to meet. Maybe a portal opens between their worlds. Start writing with an engaging introduction that sets the scene and makes the reader curious about how the crossover will unfold.
Another idea might be a crossover with One Piece. Imagine Luffy and Saitama teaming up. Luffy's elastic abilities and Saitama's one - punch power could make for some really exciting adventure and fight scenes. They could be on a quest to find a special item that only their combined skills can obtain.
Garou from OPM might also work well. His ideology of being a 'human monster' and his constantly evolving strength could make him a formidable foe for Saitama. Their clash could be not only about physical strength but also about their different views on what it means to be a hero or a monster.
Another interesting concept is a crossover where the villains from the OPM world are a threat to Saitama's own world. Saitama then has to team up with the OPM heroes to fight back. This could lead to some really cool fight scenes and interactions between different characters.
One popular OPM crossover fanfiction could be an OPM and One Piece crossover. In this type of fanfic, characters from OPM might find themselves in the world of One Piece, facing off against pirates and dealing with the mysteries of the Grand Line. Another could be an OPM and Naruto crossover, where Saitama's overpowered abilities contrast with the ninja world's complex jutsu system. There are also OPM and Dragon Ball crossovers, which often explore how Saitama's strength compares to the Saiyans' power levels.
Begin by creating a unique setting that combines elements of both universes. It could be a city that has both the chaos of the One - Punch Man world and the unique architecture or technology of the other anime's world. Then, introduce the conflict. Maybe there's a threat that endangers both worlds, and the characters from both universes must work together to stop it. Outline your story first, with a clear beginning, middle, and end, and then start writing the actual fanfiction.
The Mask's influence on the RWBY world might also change the appearance of things. If Weiss were to encounter the Mask, it could transform the elegant and icy settings she's used to into something more zany. The ice sculptures in her family's manor could start dancing around like in a wacky cartoon. And the Grimm, the dark creatures in RWBY, might react in strange ways to the presence of the Mask - perhaps they would be both terrified and strangely attracted to it at the same time.
RWBY crossover RWBY fanfiction can enhance the universe by exploring new relationships. For example, characters who have never interacted much in the original series can form new bonds. This gives a deeper understanding of the characters' personalities. It can also expand on the lore. By mixing different elements from within the RWBY world, new aspects of the history, magic, and technology can be revealed.