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.
There's a good chance he'd meet Odin. As the all - father in Norse mythology, Odin might have some grand plan for Percy. He could see Percy as a unique addition to Valhalla and offer him a special quest or task that would involve using Percy's skills and knowledge from the Greek mythological world.
Percy might meet an old enemy who has somehow been resurrected or changed in the future. This enemy could be more powerful and have a new plan to take over the world or destroy the demigods. Or he could meet a group of time - traveling demigods from an even further future who are trying to fix something in Percy's future that has gone wrong.
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.
He could meet some Aboriginal heroes or spirits. They might have their own powers and knowledge that Percy can learn from.
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.
He might meet a new teacher who is secretly a minor god or goddess in disguise.
He might meet some talented witches and wizards. Maybe there would be a student who is an expert in magical beasts and they could share stories about their encounters with strange creatures.
He might meet Luke before he turned bad. Percy could try to change Luke's fate, convince him not to follow Kronos. It would be a great chance to save a friend and prevent a lot of the chaos that happened later.
He might meet other inter - galactic beings. For example, some sort of space - faring demigods from a far - off galaxy. Or he could encounter powerful entities that are the rulers of other planets.
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.
He could meet other demigods from different pantheons. Since it's a different world, there might be demigods associated with gods that Percy has never heard of before.