A possible plot twist could be that some of the Frozen characters are actually descendants of the Greek gods in disguise. Elsa could potentially be related to Hecate, given her powerful and somewhat otherworldly magic. This would change the whole dynamic as Percy and his friends would have to figure out how to deal with this new connection. It could also lead to a situation where they have to help Elsa control her powers in a more 'Greek - god - like' way, which would be a major shift in the Frozen storyline.
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.
One interesting plot twist could be when the characters from Maximum Ride and Percy Jackson universes find out they have a common enemy. Maybe a powerful ancient being that has been manipulating events in both their worlds. This would force them to team up in unexpected ways, with the winged kids from Maximum Ride using their flying abilities and the demigods from Percy Jackson bringing their unique powers to the fight.
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.
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.
Another cool plot twist could be that the two universes start to merge because of some ancient magic or a scientific experiment gone wrong. Percy and Darcy then have to work together to stop the complete chaos that ensues. This not only forces them to combine their skills but also brings in a whole new set of challenges as they deal with characters and elements from both universes colliding.
A really interesting plot twist is when the villain in the fanfiction turns out to be a character who was initially thought to be an ally. For example, in a story where the Vikings and Arendelle are working together, a seemingly helpful Arendelle noble is actually in league with a dragon - hunting group from httyd. It throws everyone into chaos and makes for a great read.
Imagine this - Percy, thinking he's on a normal quest as the son of Hera, stumbles upon Naruto's world. But it turns out that Naruto's world is a sort of prison for the Greek gods, and they have been trapped there for centuries. Naruto and his friends have unknowingly been guarding the gods all this time. Percy now has to figure out how to free the gods without causing chaos in Naruto's world, and he has to convince Naruto to help him, which is difficult since Naruto doesn't trust these new 'powerful beings' from another world.