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 But as they do, they discover that one of the Avengers has been replaced by a shape - shifter, and Batman has to figure out who it is before it's too late.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 Darth Vader is actually related to one of the Avengers in some way. Maybe there's a long - lost family connection that changes the whole dynamic of the fight. Another could be that the power source of the Infinity Stones somehow combines with the Force, creating a new and unexpected power that either Darth Vader or an Avenger has to control. It could also be a plot twist where the Avengers think they've defeated Vader, but he actually uses a Sith technique to possess one of them and turn the tables.
One possible plot twist could be that Pennywise actually has a fear of bats, which gives Batman an unexpected advantage. Another could be that Pennywise manages to possess Batman and uses his skills and resources to wreak havoc in Gotham, but Batman's consciousness fights back from within. Or perhaps there's a plot twist where the two form an unlikely alliance to fight a greater evil that is threatening both their existences.
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.
In some fanfictions, like 'The Unlikely Alliance', the plot twist is that the Hulk is actually being controlled by an unknown villain who wants to use him to take down Batman. Batman has to figure out not only how to stop the Hulk but also who is behind it all.
In a Batman vs Avengers fanfic, an exciting plot point could be Batman's infiltration of the Avengers' base. He could use his stealth skills to get in unnoticed and then plant some sort of device that disrupts their powers or communication. Another plot point could be a moral dilemma. Maybe Batman has to decide whether to side with the Avengers against a greater threat or continue to oppose them because of his own principles. And the idea of Batman creating a custom suit specifically designed to take on the Avengers' powers would also make for an exciting plot.
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 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 fanfictions, a plot twist could be that Ghost Rider is actually a key to unlocking an ancient power that the Avengers need to defeat a new, extremely powerful enemy. This changes the whole dynamic as the Avengers have to protect and work with Ghost Rider in a different way.
In some fanfics, a great plot twist is when Ghost Rider turns out to be the key to defeating an enemy that the Avengers thought was unbeatable. His hellish powers were the one thing that could counter the enemy's invulnerability.