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.
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.
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.
Well, perhaps he could die sacrificing himself to save his friends from some ultimate evil. Then, his rebirth could be linked to a powerful ancient magic. Maybe a forgotten god decides to bring him back for some grand purpose, like to stop a new threat emerging in the world of the gods.
A possible plot in such a fanfic could be Percy trying to bring Sally back from the dead. He could go on a dangerous and forbidden quest, defying the gods and the natural order. This would put him at odds with many, but his love for his mother is so strong. He might encounter dark magic or make deals with shadowy figures in his desperation, all while his friends are worried about him losing himself completely in this quest.
In this fanfiction, it could be that Artemis is in a situation where she is about to be killed or lose her powers forever. Percy, being the selfless hero he is, takes on a deadly task to prevent that from happening. Maybe it involves facing an ancient and malevolent god who has a vendetta against Artemis, and Percy uses his unique abilities and the element of surprise to try and stop this god, but at the cost of his own life.
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.
The plot could center around Percy's struggle to fit in at college while still being a hero. He may find that the other students don't understand his need to disappear suddenly when there's a monster attack. He could also get involved in some sort of college - wide mystery that has roots in the ancient world. Like, strange symbols start appearing all over campus that only he can decipher because of his knowledge of Greek and Roman mythology. And he has to team up with some unlikely allies, like a nerdy professor who turns out to have a hidden knowledge of the demigod world, to solve the mystery and save the college from an impending doom.
Well, one possibility could be that Percy is framed for a crime he didn't commit. Maybe some powerful enemy uses magic or trickery to make it seem like he's violated a major demigod law. So, he's sent to a special prison for demigods. Inside, he has to deal with all sorts of other dangerous inmates, some of whom might be old enemies or just generally bad - tempered demigods. He'd probably try to find a way to prove his innocence and escape while also having to survive the various challenges within the prison.
One possible plot development could be that when Percy dies, his friends are devastated. They might embark on a quest to find a way to bring him back. In the process, they discover some long - hidden secrets about the gods. Another development could be that Percy's rebirth gives him new powers, which he then uses to fight a greater threat that was emerging even before his death.
The plot could involve Percy arriving early to camp and immediately noticing that something is wrong because Annabeth is missing. As he searches for her, he stumbles upon evidence that she has been in a battle with a powerful new enemy. When he finally finds her lifeless body, it shatters him. He then decides to train harder than ever before to avenge her death. He forms a small group of trusted friends from the camp and they set out to track down the enemy responsible, all while Percy is struggling with his grief and guilt for not being there to protect her.