He could meet some Aboriginal heroes or spirits. They might have their own powers and knowledge that Percy can learn from.
There could be some unique sea creatures that can transform into human - like forms in the Australian waters. Percy, being a son of Poseidon, might meet them and either make friends or foes depending on their intentions.
Percy might encounter Australian demigods. These demigods could have different abilities compared to the ones he knows in the US, and they could team up with him on a new adventure in Australia.
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.
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.
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.
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 some native Alaskan spirits. These spirits could be like the nature spirits he has encountered before but with their own unique Alaskan characteristics.
Percy might encounter Baba Yaga. She is a well - known figure in Russian folklore. She could be an adversary, testing Percy's skills and his knowledge of the magical world. Or perhaps she could be a source of information if Percy manages to earn her respect. He could also meet the Rusalki, water nymphs from Russian mythology. Given Percy's connection to water, it would be interesting to see how he interacts with them.
He might meet a new teacher who is secretly a minor god or goddess in disguise.
He would probably meet Eragon. Eragon is a key figure in Alagaesia, and their encounter would be full of surprises as they come from different worlds with different powers.
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.
Some of the more traditional and rule - following demigod groups at Camp Half - Blood could be against him. They believe in following the established ways and Percy's rogue behavior goes against that. They might see him as a troublemaker and try to bring him back to the 'right' path.