Maybe the Animus technology in Assassin's Creed could be a factor. A demigod somehow gets access to an Animus and starts reliving the memories of an ancient Assassin who had a connection to the gods. This leads to Percy and his friends having to figure out what this connection means and how it relates to a current threat. For example, a new monster from Greek mythology is being summoned by a cult that the Assassins have been tracking, and the knowledge from the Animus is the key to stopping it.
One possible plot could be that the Assassins discover a hidden artifact that has ties to the Greek gods. Percy and his friends from Camp Half - Blood get involved because they sense the godly power in the artifact. They team up with the Assassins to protect the artifact from a common enemy, like a group of rogue demigods who want to use the artifact's power for evil. The Assassins' skills in stealth and combat combined with the demigods' powers would make for some exciting action scenes.
One possible plot could be that the protagonist from Assassin's Creed Odyssey somehow gets transported to the world of PJO. Maybe they end up in Camp Half - Blood and are mistaken for a demigod at first. They then have to use their assassin skills to survive the various challenges in this new world, like facing off against mythical creatures that are different from the ones in their own world.
The assassins from Assassin's Creed Odyssey are very stealthy and skilled fighters. When they meet the demigods from PJO, they might initially be cautious. For example, Kassandra or Alexios could befriend Percy. Percy would be impressed by their combat skills, while the assassin would be amazed by Percy's ability to control water. They could learn from each other's fighting styles.
One idea could be Percy having to face a new and powerful monster that is a combination of different mythological creatures. For example, a chimera with the head of a hydra. This would create a unique and challenging battle for him.
The Assassins, being trained in combat and stealth, would initially be a bit skeptical of the demigods' powers. For example, Ezio might be quite surprised when he sees Percy control water to put out a fire in a narrow alley during a fight. On the other hand, the demigods would be fascinated by the Assassins' gadgets and skills. Annabeth might be very interested in the Assassins' hidden blade mechanism and try to figure out how it works. They would learn from each other, with the Assassins teaching the demigods about urban warfare and the demigods showing the Assassins how to use their powers more effectively in combat.
A cool plot could be that the Templars in the Assassin's Creed world somehow get hold of some ninjutsu scrolls. They start using these to create an army of super - soldiers. The Naruto characters then team up with the assassins to stop the Templars. Naruto could use his Rasengan against the Templars' new warriors, while Altair could use his eagle vision to spot their weaknesses. Together, they could form a unique combat strategy that combines the best of both worlds.
One possible plot could be that Percy and his demigod friends are somehow transported to the Halo universe. They could team up with the Spartans to fight a common enemy, like a new kind of powerful monster that has emerged in the Halo world due to some strange rift between the two universes. Percy could use his water powers in unique ways, for example, to short - circuit some of the Covenant technology which might be vulnerable to water in this crossover scenario.
One possible plot could be that Kratos, from God of War, somehow ends up in the Assassin's Creed universe. He teams up with an Assassin, perhaps Ezio. Together, they face a common enemy that threatens both their worlds. This enemy could be a powerful Templar who has acquired some of the dark magic from Kratos' world.
In a possible Assassin's Creed Watch Dogs crossover fanfiction plot, the modern - day setting of Watch Dogs is somehow intertwined with the historical conspiracy elements of Assassin's Creed. For example, the main character, a hacker, starts to receive encrypted messages that seem to be related to the Assassin's Brotherhood. As he delves deeper, he realizes that there are hidden forces at work in the city that are much older and more powerful than he thought. He then meets an Assassin who has been secretly operating in the city for years. Together, they uncover a plot that involves a powerful corporation trying to use ancient Assassin technology for their own malicious purposes. The Assassin teaches the hacker some of the ways of the Brotherhood, like how to read hidden signs in the city, and the hacker shows the Assassin how to use modern technology to their advantage.
One possible plot could be that the Assassins discover a strange and otherworldly artifact that is somehow linked to the power of Ghost Rider. The main Assassin character, perhaps Ezio or Altair, starts to have strange visions related to the Ghost Rider's world. They then have to figure out how to use or contain this power before it falls into the wrong hands, like the Templars. Along the way, they meet a character who has some connection to the Ghost Rider, like a descendant or someone who has been touched by the same dark magic.
The plot could involve Percy being bullied by a new group of monsters in disguise at Camp Half - Blood. These monsters, looking like normal campers, could be trying to weaken Percy before a big battle. Percy would start to doubt himself because of their constant taunts and pranks.