There could be a sidekick character, like a little alien creature named Zee. Zee might be small but is really smart and helps Percy out of many tricky situations during their travels.
Well, in some of the adventures, Percy might encounter strange alien species. For example, he could meet a race of telepathic beings who communicate through colors. He has to learn their unique language to get help on his journey through the universe.
Annabeth could definitely be one of his allies. She's smart and resourceful, and her knowledge of architecture and strategy would be useful no matter where in the universe they are. She could help Percy figure out the layout of strange alien cities or fortresses.
In this fanfiction, Percy might encounter alien species with unique powers. For example, he could meet a race that can control time, which would lead to some really exciting adventures as he tries to navigate through their time - warped worlds while also dealing with his own demigod powers.
He could meet his future self. This encounter might be full of surprises as his future self could have different views and experiences. His future self might be more jaded or more powerful.
One possible plot could be that Percy discovers a magical device that sends him to the future. There, he might find a world that has been drastically changed by the actions of the gods in his time. Maybe the demigod camps have evolved into something completely different, and he has to adapt to new rules and powers.
Percy is the central figure in 'Percy Travels the World Fanfiction'. Alongside him, if the fanfiction stays true to the original series, characters like Tyson could show up. Tyson, being a cyclops, could offer his strength in dangerous situations. And we can't forget about Chiron. He might be guiding Percy from afar, giving him advice through some magical means. There could also be characters unique to the fanfiction, perhaps a powerful witch in a European forest who becomes an ally or an adversary to Percy.
Well, he might join the Avengers. Imagine Harry using his magic skills alongside Iron Man's tech. It could be a really cool combination. He could use spells to enhance the team's defenses or help in battles in ways the regular Avengers couldn't. Maybe he'd have some clashes with Doctor Strange at first, since they both deal with magic, but then they'd probably end up working together.
In this fanfiction scenario, Naruto could bring his ninja skills to the Def universe. He'd probably have to adapt to the new environment. For example, if the Def universe has different laws of physics, his usual jutsu might not work the same way. He might also learn new techniques from the Def universe and combine them with his own.