Perhaps there's a magical artifact that has been stolen from both the Greek and Quileute worlds. Percy and Jacob are both on the hunt for it. They cross paths and at first, they fight over who should have the artifact. But then they figure out that the real mastermind behind the theft is someone much more powerful. So they join forces, Percy using his sword - fighting skills and Jacob using his werewolf strength and speed to retrieve the artifact and save both their worlds from chaos.
One idea could be that Percy and Jacob meet in a strange dimension where the rules of their respective worlds are all jumbled up. They have to team up to find a way back to their own realms while facing challenges specific to both their mythologies, like Percy dealing with a werewolf - themed water monster and Jacob having to navigate through a maze filled with Greek demigods' traps.
Perhaps there's a villain who is stealing both Greek and Wakandan artifacts. Percy and the Black Panther have to team up to stop this villain. They could combine their skills, with Percy using his demigod powers to fight off mythical creatures the villain summons, while the Black Panther uses his combat skills and vibranium - enhanced suit to take on the human henchmen. Along the way, they learn about each other's cultures and grow to respect them more.
One idea could be that Hermione is on a research mission in the forest where Jacob Black's pack roams. Their paths cross and at first they're wary of each other. But as they start to communicate, they realize they can help each other. Jacob can provide Hermione with knowledge about the forest's magic that she didn't know, and Hermione can offer her magical expertise to solve some problems the pack is facing, like a mysterious dark magic that's affecting the forest animals.
One plot idea could be Percy having to team up with Hera to rescue a group of demigods who have been captured by an ancient monster. Hera, despite her usual aloofness, realizes she needs Percy's unique skills.
I really like the fanfiction 'Mystic Alliances'. In this one, Percy's powers interact in an interesting way with Jacob's wolf abilities. The story has a great plot that keeps you hooked from start to finish. It also develops the characters in a new light, showing how they grow and adapt in a combined universe.
One idea could be that Rosalie and Jacob are forced to work together on a mission to protect the town from a new supernatural threat. They start off hating each other but gradually come to respect one another's skills.
One idea could be Jacob and Renesmee going on a journey to discover more about Renesmee's unique abilities. They could encounter other supernatural beings along the way who either help or hinder them.
One idea could be Percy and his friends getting trapped in a virtual game world that's a twisted version of their own. They have to use their demigod skills to defeat the digital monsters and find a way out.
Maybe Percy is sent on a quest to Atlantis because there are rumors of a traitor among the Atlanteans who plans to side with the Titans. Percy has to navigate the complex political situation in Atlantis and figure out who the traitor is before it's too late. He also has to deal with the Atlantean magic which is different from what he's used to.
One idea could be that Percy discovers a hidden realm within the world of the Wanderer. This realm has its own set of rules and challenges that he has to navigate. Maybe he meets new demigods there who have been lost for centuries.
One idea could be Percy having to make a deal with a minor underworld god to save Annabeth from a new and strange curse that's slowly draining her life force. He has to complete a series of tasks in the underworld that test his loyalty and courage more than ever before.