Well, perhaps it could be about Annabeth's initial hesitation to join the Hunters. Maybe she has some personal goals in Camp Half - Blood that she's reluctant to leave behind. For example, her relationships with Percy and her friends there.
It could be an old enemy of the Hunters. Maybe a powerful monster that has a grudge against them for past defeats. For instance, a giant who has been waiting for the right time to strike back at the Hunters.
The story of 'percy joins the hunters fanfiction' could start with Percy being invited to join the Hunters for some special reason. Once he joins, he has to deal with the fact that the Hunters are mostly female and have their own traditions. He may find himself in the middle of internal Hunters' politics. For example, some Hunters might be skeptical about his presence, while others see his potential. His journey within the Hunters could also lead to self - discovery, as he learns more about himself and his place in this new group, all while going on quests and battling against various foes.
The plot could start with female Percy feeling a bit out of place in her normal world. She hears about the Hunters and is intrigued. She decides to join them. Once she joins, she has to learn their ways, like how to use their special bows and arrows better.
One possibility is that Percy has to face a new set of challenges related to the Hunters' code. For example, he might struggle with the no - male - relationships rule.
One possibility is that Annabeth has made a mistake that hurt her friends in the demigod world. Maybe she misjudged a situation during a quest and led the team into danger. So, she apologizes for her error in judgment and tries to make it up to them by coming up with a new plan to succeed in the quest.
Well, one possibility could be that Naruto, with his ninja skills and strong sense of justice, joins Slade for a common goal like fighting a greater evil that neither could face alone. Maybe they team up to take on a powerful, shadowy organization that threatens both their worlds.
Well, perhaps Harry could discover a magical portal that leads him to the Protoss world. There, he might learn their advanced technology and unique culture. Maybe he uses his magic in combination with Protoss powers to fight off some common enemies.
Well, one possible plot could be that Naruto, disillusioned with the Hidden Leaf Village after some major betrayal. He might start to see the Akatsuki's goals in a different light. For example, if the village wrongly accuses him of a crime he didn't commit, like they did with Sasuke in some ways. He could then be approached by an Akatsuki member who offers him a place in the group where he feels he can be understood. And Naruto, in his search for a new purpose and a place to belong, decides to join.
Well, one possibility could be that Harry, through some sort of magical accident or powerful spell, gets transported to the world of Loki. Maybe he initially mistakes Loki for a powerful wizard and tries to seek his help to get back home. Loki, being the trickster he is, might see Harry as an interesting pawn in his schemes against the Asgardians or other foes. Harry, with his own magical skills from the wizarding world, could end up being both a hindrance and a help to Loki, leading to all sorts of adventures and misadventures as they navigate their two very different magical realms.
One possible plot could be that Sakura becomes disillusioned with Konoha for some reason. Maybe she discovers some dark secrets or feels betrayed. So, she decides to join the Akatsuki in search of a new purpose. Once there, she has to deal with the other members, some of whom might be suspicious of her motives at first.