If Percy Jackson goes to Valhalla in fanfiction, it could be a really interesting crossover. He'd be out of his normal Greek mythology setting. He might find the Norse afterlife very different from what he's used to in the Underworld. For example, the feasting and fighting in Valhalla would be a new experience for him. And he could share his own adventures from the Greek world, which would surely amaze the Norse inhabitants.
Well, he might meet some really tough Norse warriors. Maybe he'd get into some epic battles or challenges. He could also make new friends or enemies among the Norse gods and heroes there.
If Percy Jackson goes to Valhalla in fanfiction, he could meet a whole bunch of Norse heroes. There's Sigurd, the dragon - slayer. And then there are the Valkyries who choose who gets to go to Valhalla. He might also run into Loki, who is always causing some sort of trouble. Percy would be in for quite an adventure as he tries to figure out who he can trust among these new and often unpredictable characters.
Well, if Percy goes to the past in fanfiction, he could encounter younger versions of the gods. Maybe he would befriend a young Zeus or Poseidon before they became so powerful and full of pride. He could also learn new and forgotten magic or fighting techniques that were lost over time.
Percy might run into Loki. Loki is known for his trickery. He could either befriend Percy for his own devious plans or try to deceive him. Percy, being used to dealing with tricky situations in his own world, would have to be on his guard against Loki's schemes.
If Percy Jackson goes to protect Hogwarts in fanfiction, it'd be an amazing crossover. Percy is used to facing all kinds of monsters, and at Hogwarts, he'd have to adapt to magic. He might form an alliance with some of the Hogwarts students. His speed and combat skills could help in protecting the school from any invaders, whether they be Death Eaters or other dark creatures.
Well, one possibility could be that Percy discovers a new aspect of the mythological world. Maybe in this fanfic, his death is a sacrifice to save his friends. Once in Valhalla, he could meet new heroes from different mythologies and form alliances or rivalries. He might also try to find a way back to life or to his friends in the world he left behind.
If Percy Jackson goes to UA, there would be a huge culture clash at first. The UA students are used to a very structured hero training system, while Percy is more of a 'go with the flow' kind of demigod. But over time, he could teach them about improvisation in battle and they could teach him about teamwork in a more organized way. He might also introduce the concept of Greek mythology to the UA world, which could open up new ways of thinking about powers and abilities.
In a fanfiction where Percy Jackson goes to Forks, he might be really confused by the gloomy weather. He's used to more sunny and open places. He could meet the Cullens and there would be a clash of his demigod powers and their vampire abilities. Maybe he'd mistake them for some new kind of monster at first.
Perhaps in this fanfiction, Percy goes to Australia to find a hidden artifact that is crucial for the safety of the demigod world. Along the way, he teams up with local demigods (if there are any in the fan - created story). He would have to adapt to the Australian culture and environment, which is very different from what he's used to in the US. He might also face some new types of monsters that are native to Australia.
In this fanfiction, Percy might decide to break away from Camp Half - Blood. He could start exploring the mortal world on his own terms, using his demigod powers in ways that the camp rules don't allow. Maybe he teams up with some unexpected allies, like a group of outcast demigods or even some friendly monsters.
Well, he might meet Elsa and be really amazed by her ice powers. Since Percy is used to dealing with all kinds of magic in his own world, he could either help Elsa control her powers better or get into some trouble because of the different nature of the magic in Arendelle. Maybe he'd also team up with Anna to fight against some new threat that emerges in Arendelle.