Well, a plot twist could be that the modern army's weapons are actually ineffective against the fantasy creatures because they are made of some sort of magical substance that absorbs or deflects the bullets and missiles. Then, a lone scientist in the modern army discovers a way to combine magic and technology, creating a new weapon that can defeat the fantasy side. Another plot twist could be that the fantasy side has a prophecy that a group of modern soldiers will save their world, so instead of fighting, they welcome the modern army and ask for their help against an even greater evil within their own realm.
One possible plot twist could be that Hinata discovers a new power within herself during the fight against Pain. Maybe she can tap into some ancient Hyuga ability that even she didn't know existed. This new power could turn the tide of the battle and surprise both Naruto and Pain.
Well, a great plot twist could be that Batman discovers that Naruto is being controlled by an evil force. Instead of fighting Naruto, he decides to help him break free from the control. This would lead to an unexpected alliance between them. Another plot twist could be that the fight between them is actually a test set up by some higher - level beings from both universes. They are being watched and judged based on how they fight and interact. And if they pass the test, they are given a special power or mission. Or maybe there's a plot twist where Naruto's friends from Konoha are kidnapped and held hostage by someone who wants Batman to defeat Naruto. Naruto then has to decide whether to fight Batman or try to find another way to save his friends in this fanfic scenario.
It really depends on the specific circumstances. If the fantasy side has powerful magic, like being able to conjure up huge firestorms or turn the ground into quicksand under the modern army's feet, they could win. But modern armies have advanced weaponry like missiles and tanks. If they can quickly figure out the weaknesses of the fantasy elements, they might prevail.
One exciting plot twist could be that Venom actually has a change of heart. Maybe he realizes that he and Spider - Man are both victims in a way, and they end up teaming up against a bigger threat. Another could be that Spider - Man gets infected by a different symbiote that makes him even more powerful, but also more unpredictable, and now Venom has to decide if he wants to fight this new version of Spider - Man or join him.
The modern army would have firearms, missiles, and advanced explosives. These weapons are highly destructive and can cause mass damage from a distance. In contrast, the fantasy army would likely have swords, bows, and magic spells. The modern weapons would seem overpowering at first, but the magic in the fantasy army could potentially counter them. For example, a magic shield could deflect bullets.
One interesting plot twist could be that Goku and Beerus end up teaming up instead of fighting. Maybe they discover a common enemy that is even more powerful than them. Another could be that Beerus loses his power temporarily and Goku has to protect him from other villains while also trying to figure out how to return Beerus' power.
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.
Maybe in the fanfiction, Naruto loses his chakra early on in the fight against Pain. But then he discovers a hidden power within himself that is not related to chakra, like a spiritual power from his Uzumaki heritage, which he uses to turn the tables on Pain.
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.