Well, we could have a situation where the Monster Association from One Punch Man discovers Equestria. They think it's a new place full of potential 'prey'. The ponies, not used to such brutal threats, would be in a panic. Then Saitama follows the monsters to Equestria. Along the way, he befriends some of the ponies, like Pinkie Pie who tries to cheer him up all the time. The ponies and Saitama would then work together to drive the monsters out of their land. It would be a really interesting combination of the two very different universes.
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 crossover with 'Dragon Ball' would be cool. Picture Saitama facing off against Goku. It would be an interesting contrast between Goku's love for a good fight and Saitama's boredom with opponents who can't challenge him.
How about a crossover with 'Attack on Titan'? Saitama could find himself in the world of the Titans. His ability to defeat any enemy with one punch would be extremely useful against the Titans. He might not understand the complex politics and fear that surrounds the people in that world at first. But as he starts taking down Titans left and right, he would become a sort of legend among the survivors.
Another plot could involve a crossover with a different anime universe through a strange portal. For example, if he entered the world of My Hero Academia. Garou could initially try to assert his dominance but quickly realizes that the quirks in that world are different from his martial arts skills. He might then start to learn about these new powers and how to adapt, all while causing chaos and also forming unexpected alliances.
Perhaps there could be a villain in the crossover fanfiction that is a threat to both worlds. Saitama and Ainz have to team up to defeat it. Saitama is used to soloing enemies, and Ainz is used to having his minions do the work. So, they have to overcome their differences in fighting styles and learn to work together. Ainz might be in awe of Saitama's sheer strength, and Saitama might be a bit weirded out by the undead nature of Ainz and his minions.
Another fanfiction concept could be that the monsters in the 'Danmachi' dungeon start to mutate in a way similar to how the monsters in 'One Punch Man' are strange and powerful. This would force the adventurers in 'Danmachi' to come up with new strategies, and perhaps some of the heroes from 'One Punch Man' could come to help out, like Genos, who could provide some technological insights on dealing with these new threats.
One idea could be that Saitama's hair gives him some new, unexpected power. For example, his hair could start to glow and act as a power amplifier, allowing him to defeat enemies even more easily than before. This would be a fun twist on his already overpowered character.
One idea could be that Godzilla somehow ends up in Equestria. The ponies are initially terrified but then realize Godzilla is more interested in protecting the land from some new dark force that has emerged there. Maybe Godzilla's size and power are a mystery to the ponies at first, and Twilight Sparkle tries to study him to figure out how he got there.
A possible plot could involve a dark force threatening both the world of Lucario and Equestria. A group of ponies and Lucario have to come together. The ponies use their magic and the Lucario use their combat skills and aura powers to fight off the common enemy. This would show how different abilities can be combined in a crossover story.
One idea could be that Thomas the Tank Engine somehow ends up in Equestria. Maybe there's a magical portal that transports him there. He could then interact with the ponies, perhaps helping them with some big construction project like building a new bridge. The contrast between his mechanical world and the magical world of MLP would create a lot of comical and interesting situations.