One interesting plotline could be the characters from different parts of Percy Jackson's world suddenly finding their lives intertwined in unexpected ways. For example, demigods from different camps having to team up against a new, powerful threat that targets all of them regardless of their origin.
Percy Jackson himself is very likely to be one of the main characters. His life is at the center of much of the action in the original series, so in a fanfiction about lives colliding, he would be a key figure. Annabeth Chase would also be likely involved, as she is Percy's close companion and often involved in his adventures.
Annabeth Chase could also be a main character. She's Percy's close friend and often involved in his adventures, so if lives are colliding in the fanfic, she'd be right there in the middle of it all. Additionally, Grover might be a main character too, as he's Percy's best friend and has his own important role in the Percy Jackson world.
One possible interesting plotline could be the unexpected meeting of different demigods whose lives were on separate paths before. For example, characters from different cabins in Camp Half - Blood coming together to face a new and powerful enemy that requires their combined skills.
The worlds could collide through a portal. Maybe a demigod accidentally creates a portal while trying out a new spell. This portal leads to the world of the Transformers. Percy Jackson and his friends are then sucked into this new world. They find themselves in the middle of the war between Autobots and Decepticons and have to choose a side or find a way to create peace between the two factions.
Well, in a fanfiction scenario, the immortal Percy Jackson could bring his demigod powers to the Avengers' world. Maybe his water - controlling abilities could help in battles against villains who use fire - based attacks. For example, against a foe like Ultron, Percy could douse his mechanical parts and disrupt his systems.
In this fanfiction, Percy living alone could mean a whole new set of adventures. Maybe he's chosen to isolate himself after all the chaos in the demigod world. He could be exploring his powers in solitude, away from the distractions of Camp Half - Blood. It also might lead to some self - discovery as he doesn't have to rely on his friends all the time.
Well, one way their worlds could collide is through a common enemy. Maybe some ancient Greek monster has found a way into the modern world of the Avengers. Percy, with his water - controlling powers and combat skills, could team up with Thor, who also has power over a natural element. They would make a formidable duo against this threat.
One way their worlds could collide is through the concept of magic. In the Percy Jackson universe, there's Greek magic, and Doctor Strange is a master of the mystic arts. Maybe Percy could end up in the Sanctum Sanctorum by accident and Strange has to figure out how to send him back to his world. It would be a wild adventure as Percy's demigod powers might react strangely to the magic in Strange's world.
One way the worlds could collide is through a magical accident. Maybe some of the Infinity Stones' power messes with the magic in Percy Jackson's world, causing portals to open. Percy and his friends could be sucked into the Avengers' universe right in the middle of the Civil War. They might side with different factions based on their own beliefs about power and control.