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.
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.
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.
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.
Another great crossover would be with One Piece. Saitama could be on a journey with the Straw Hat Pirates. His super strength would come in handy during battles against the various pirates and marines they face. And his comical personality would contrast well with the wacky and diverse personalities of the Straw Hat crew.
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.
Begin with researching the two universes you are using for the crossover. Know the rules, the characters, and the settings well. For a saitama crossover with the Attack on Titan world, you need to understand the Titans, the Survey Corps, and the whole concept of the walls. Then, introduce Saitama in an interesting way. Maybe he appears right in the middle of a Titan attack. His presence would disrupt the usual order of things, and his simple approach to fighting the Titans (just punching them) could either amuse or terrify the characters in that world.
One idea could be that Saitama somehow ends up in the RWBY world. He could be mistaken for a very powerful Huntsman. Maybe he decides to help out at Beacon Academy, but his non -chalant attitude towards the Grimm would be really amusing. For example, he might just one - punch a huge Grimm and then be like 'Is that it?'