One unique plot twist could be that Saitama suddenly loses his power and has to learn volleyball skills from the Haikyu characters to regain his strength. Another might be that a powerful monster in One Punch Man is actually a huge volleyball fan and spares the Haikyu characters because he wants to see them play.
A really cool plot twist I've seen is that the spirit of volleyball in Haikyu is so strong that it actually gives the One Punch Man characters new abilities. For instance, Genos discovers that by playing volleyball, he can upgrade his parts in a different way than he's used to. And on the other side, the Haikyu characters find that they can use some of the One Punch Man hero's techniques in their volleyball games. This unexpected exchange of abilities and how they change both sets of characters makes for a really engaging and unique fanfiction plot.
One great Haikyu and One Punch Man fanfiction could be where the characters from Haikyu find themselves in the world of One Punch Man. They might be shocked by Saitama's incredible strength. Another could be a crossover where the volleyball skills of the Haikyu characters are used in a unique way in the One Punch Man universe, like to defeat some of the monsters in a really creative way.
One unique plot idea is the reversal of roles. In some fanfics, the villains become the heroes and vice versa. It gives a whole new perspective on the 'One Punch Girl' universe.
Well, in some fanfictions, there might be a plot twist where Geryuganshoop is actually from a different dimension and has a mission that is completely different from what we initially thought. He could be trying to protect Earth in his own strange way, which goes against his initial villain - like appearance. This would lead to a lot of interesting interactions with the other characters in One Punch Man.
One interesting twist could be that a Titan eats a character from One Punch Man and gains not only their physical abilities but also their memories. This Titan then starts to have an identity crisis and tries to rebel against the other Titans. It could lead to a situation where the heroes have to decide whether to kill this unique Titan or try to use it to their advantage.
One common plot twist could be that the reincarnated character has some unique power that even Saitama is interested in. For example, they might have a power that can nullify Saitama's strength temporarily. Another plot twist could be that the character is reincarnated as a monster but still retains their human consciousness and tries to find a way to co - exist with humans. Also, a plot twist might be that the reincarnated character discovers a hidden conspiracy in the One Punch Man world that even the heroes don't know about, like a secret organization controlling the appearance of monsters.
Well, one possible plot twist could be that Saitama's boredom turns into a malicious form. Since he's so powerful and always bored from easily defeating enemies, this new evil side might start creating stronger foes just to have a real challenge, but without caring about the destruction it causes. He could also turn against the other heroes, seeing them as weaklings not worthy of his time anymore.
Another plot twist might be that there is a hidden alliance between some subterranean and a group of surface - dwellers. They are working together in secret to achieve a common goal, perhaps to take down a powerful and corrupt organization. This adds a layer of mystery and conspiracy to the fanfiction.
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.
A really unique plot twist can be found in 'The School that Changed Natsu'. Here, the school Natsu attends is actually a magical illusion created by an enemy. The whole school experience is a trap to keep Natsu and his friends away from Fairy Tail during a crucial time. Natsu has to figure out the truth and break free from this false reality, which leads to some intense and exciting scenes.
One exciting plot twist could be Saitama suddenly losing his one - punch power in the Naruto universe. This would force him to adapt to new ways of fighting, like using chakra. Another could be Naruto learning Saitama's training regime and attempting it, only to find it's way beyond his physical limits. And perhaps the villains from both universes team up, making it extremely difficult for the heroes to combat them.