In a Saitama vs OC fanfiction, a plot twist could be that the OC has the power to make Saitama lose his strength. However, the OC doesn't want to use this power because they admire Saitama and want to test their own strength against him fairly. Another interesting twist could be that the OC is a part of a bigger plan by some mysterious organization. They use the fight against Saitama as a distraction while they carry out their real objective elsewhere. And as the fight progresses, Saitama realizes this and has to decide whether to continue the fight or go after the real threat.
One possible plot twist could be that Jiren discovers a way to nullify Saitama's strength. Maybe he finds some sort of energy - draining technique that weakens Saitama over time. Another could be that Saitama holds back for some reason, like he wants to see the full extent of Jiren's power, and in doing so, he gets into a real struggle.
One interesting plot twist could be that Brightburn is actually from the same planet as Superman but was sent to Earth for a different purpose. Maybe he was an experiment gone wrong. Another could be that Superman's powers start to fade during the fight, making it a more even battle. Or perhaps Brightburn is being controlled by an unseen force that Superman has to figure out how to stop while also fighting him.
In some Batman vs Avengers fanfiction, a great plot twist is when Batman manages to hack into the Avengers' communication system and makes them fight among themselves. He then uses the chaos to his advantage. Another cool twist is that there's a greater threat looming, and Batman and the Avengers have to put aside their differences and work together. But as they do, they discover that one of the Avengers has been replaced by a shape - shifter, and Batman has to figure out who it is before it's too late. There could also be a plot twist where Batman uses a device that actually amplifies his own strength and skills to the point where he can match the Avengers' superpowers, at least temporarily.
A really cool plot twist would be if Batman manages to steal all of Thanos' Infinity Stones and use them against him. This would completely turn the tables. Or perhaps there's a plot twist where Batman uses his detective skills to uncover a secret weakness of Thanos that no one else knew about. Maybe it has something to do with Thanos' past or his family, and Batman uses this knowledge to his advantage in the fight.
One interesting plot twist could be that Ultron hacks into Spider - Man's suit. This would put Spider - Man at a huge disadvantage as Ultron could control his movements or disable some of the suit's functions.
One interesting plot twist could be that Percy discovers a way to nullify Kratos' god - given powers. Maybe he finds an ancient artifact during the fight that has the ability to do so. Another could be that Kratos, instead of seeing Percy as an enemy, realizes they have a common goal against a greater evil in the fanfiction world and they end up teaming up.
A great plot twist would be if the Fairy Tail guild members were transported to the Naruto world and had to adapt to ninja life. Or perhaps a character from Naruto, like Sasuke, gets corrupted by the dark magic in the Fairy Tail world. It would create a whole new dynamic as the other characters try to save him. Also, what if there was a prophecy in both worlds that the only way to save them was for a Fairy Tail wizard and a Naruto ninja to sacrifice themselves? That would be a very intense plot twist.
One interesting plot twist could be that Naruto is actually related to a Marvel character through some ancient connection. Maybe his chakra has a link to the Infinity Stones in the Marvel universe. Another could be that the Avengers are transported to the Naruto world and have to learn ninjutsu to survive. And a third one might be that Naruto gets the powers of a Marvel hero like Spider - Man and has to deal with the new abilities while still being a ninja.
A great plot twist could be that they are both being manipulated by a third force. They think they are fighting each other for their own reasons but in reality, they are being used. Maybe a god or a powerful demigod is pulling the strings. This would lead to a moment of realization and then they would have to figure out how to work together to stop the real threat. Also, a plot twist where the Mark of Athena has a different meaning for each of them could be really engaging. It could be that Percy thinks it means one thing and Jason thinks it means another and their interpretations lead to a big misunderstanding before they figure it out.
One interesting plot twist could be that they find out they are related in some way through the complex web of mythology. Maybe there's a long - lost ancestor who was part of both Greek and Norse worlds. Another could be that they are forced to swap powers. Percy has to deal with Magnus's Norse magic and Magnus has to handle Percy's water powers. It would be really cool to see how they adapt.