You can make it unique by focusing on the character interactions. For example, if you cross it with 'Attack on Titan', instead of just having Saitama fight Titans, have him interact with the Survey Corps members on a personal level. Show how his nonchalant attitude affects them.
To create unique characters in One Punch Man fanfiction, consider their appearance. Make them look distinct, like a hero with multiple arms or a villain with a body made of crystal. Their motivation is also important. A character who wants to be a hero just to get famous is different from one who wants to protect a specific person. And don't forget about their weaknesses. A character with a power that has a really strange weakness, like being allergic to a certain type of flower, can be very interesting.
You can focus on character relationships. For example, if you cross Spider - Man with Iron Man, explore how Tony Stark could be a mentor figure to Peter Parker. This dynamic can add depth to the story.
Another idea is that Naruto, curious about other powerful beings, uses a jutsu to travel to the One Punch Man universe. There, he would be amazed by Saitama's strength. Naruto could try to learn from Saitama how to be so effortlessly strong, but Saitama would probably just shrug it off and say there's no special trick. They could team up to fight some of the monsters in Saitama's world, with Naruto using his ninjutsu and Saitama just punching things away.
A fanfiction might center around a competition between Popeye and Saitama. They could have a friendly contest to see who can defeat the most villains in a day. Popeye would use his wits and spinach - enhanced strength, while Saitama would rely on his incredible punching power. This could lead to some hilarious and exciting encounters as they both try to outdo each other.
There's a fanfic where Garou spares One Punch Man and then they both get transported to an alternate dimension. In this new world, they have to work together to find a way back while also facing new and strange enemies. The unique part is how their relationship develops in this unfamiliar setting.
The interactions would be quite diverse. Thor might see Saitama as a strange but powerful being. He could offer to have a friendly sparring match, which Saitama might decline because it's too much trouble. Black Widow would likely be trying to figure out Saitama's background and what makes him tick. And Spider - Man would be super excited to meet someone with such incredible power, constantly asking Saitama questions about how he got so strong.
A really cool 'One Punch Man Saitama Crossover Fanfiction' is the one that crosses him over with the My Hero Academia universe. Saitama in a world full of quirks, and how his lack of any 'quirk' but extreme physical strength is perceived by the heroes and villains there. It's a unique take on both universes and has some really funny moments as Saitama tries to fit into the MHA society.
You can make it unique by changing the character dynamics. For example, make Dracula a more sympathetic character. Maybe he was cursed into his evil ways and the fanfiction could be about his journey to break the curse. Also, introduce new species or races that are not in the original Castlevania or Dracula stories. This could add new plot twists and conflicts.
You can make it unique by changing the setting. Instead of the usual spooky locations in the US, set the crossover in an ancient castle in Europe. The Ghost Adventures crew would have to deal with different types of hauntings and superstitions.
A character could be a hair - stylist in the One Punch Man universe who is obsessed with getting to style Saitama's hair. He or she could follow Saitama around, much to his annoyance, offering different hair - styling ideas and thinking that it would enhance Saitama's already overpowered status.