Perhaps Percy's presence in the Norse world as Thor's brother starts to change the fates that were foretold. In Norse mythology, there are many prophecies. Percy's actions and his influence on Thor could start to rewrite these prophecies, causing chaos among the Norse gods. For example, a prophecy about Ragnarok might be affected, and the gods have to deal with the unexpected consequences. This would make for a very engaging and unpredictable fanfiction plot.
A great plot twist could be that the gods of both worlds had a secret plan all along. They made Percy and Thor brothers to create a bridge between the Greek and Norse realms. But when Percy and Thor start to figure this out, they face opposition from some of the more traditional gods who don't want the two worlds to mix. This could lead to a conflict where Percy and Thor have to fight not only against external enemies but also against their own godly families. It would add a whole new layer of drama to the fanfiction.
In some fanfictions, a really cool plot twist could be that Percy's Hela - given powers are slowly corrupting him. He starts to see the world in a different, darker way. His friends notice the change in him but don't know the cause at first. As he struggles with this corruption, he has to find a way to either embrace it and use it for good in a new way, or find a way to get rid of it. Also, imagine if Percy discovers that he has a half - brother or sister from Hela's side in the Norse world, and this new relative has a completely different view on life and power, which causes more drama and conflict for Percy.
In the stories, the revelation about Luke being a traitor is a big plot twist. He was initially seen as a friend and mentor to Percy. But later, it turns out he has his own dark motives and is working against the gods, which adds a lot of complexity and danger to the story.
A very creative 'Thor meets Percy Jackson' fanfiction involves the two being sent back in time to the creation of their respective mythologies. They have to make sure things happen as they should without changing history too much. It's full of interesting twists as they interact with the primordial beings of both mythologies.
One interesting plot twist could be that Percy discovers he has a long - lost sibling within the family. Maybe a half - brother or sister from a different godly encounter that his parents had in the past. This would change the family dynamics completely and add a new layer of relationships for Percy to navigate.
One interesting plot twist could be that Percy's water powers somehow interact with Elsa's ice powers in unexpected ways. Maybe when they meet, their powers combine to create a new form of frozen water element that has unique properties. Another could be that the demigods from Percy Jackson's world have to deal with the magical cold of Arendelle, which is a whole new kind of challenge compared to their usual Greek - related threats.
Perhaps a plot twist where the Mist from Percy Jackson's world starts to affect Tokyo Ghoul. The Mist could make humans in Tokyo see ghouls as something completely different, like mythical creatures from their own Japanese folklore. This would not only change how the humans in Tokyo Ghoul react to ghouls, but also how the ghouls themselves perceive their place in the world and their relationship with humans.
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.
In some fanfics, a plot twist could be that a demigod turns into a vampire. This completely changes the dynamic as they now have to deal with their new vampiric nature while still having their demigod powers. It's a really unique take on the crossover.
One interesting plot twist could be when Atalanta turns out to be a long - lost relative of one of Percy's friends. This completely changes the dynamic between them and adds a whole new layer of complexity to the story.
In some fanfics, a plot twist might be that the Death Note has some sort of connection to the Greek underworld. Maybe it was created by Hades as a way to control the souls in a different way, and when Light Yagami starts using it, he unknowingly disrupts the balance between the mortal world and the underworld. This could lead to Percy and his friends having to step in and deal with both Light and the chaos in the underworld.